Blob/File type: preserve user-supplied MIME verbatim (WHATWG File API)

src/bun.js/VirtualMachine.zig

@@ -346,7 +346,13 @@
     return this.rareData().mimeTypeFromString(this.allocator, str);
 }
 
+/// Interning lookup for Blob/File `type` that preserves the raw MIME
+/// string (no charset substitution). See `rare_data.mimeTypeInternedValue`.
+pub fn mimeTypeInternedValue(this: *VirtualMachine, str: []const u8) ?[]const u8 {
+    return this.rareData().mimeTypeInternedValue(this.allocator, str);
+}
+
 pub fn onAfterEventLoop(this: *VirtualMachine) void {
     if (this.after_event_loop_callback) |cb| {
         const ctx = this.after_event_loop_callback_ctx;
         this.after_event_loop_callback = null;

src/bun.js/rare_data.zig

@@ -319,6 +319,30 @@ pub fn mimeTypeFromString(this: *RareData, allocator: std.mem.Allocator, str: []
     return null;
 }
 
+/// Look up a MIME type string in the interned table and return its raw
+/// (uncanonicalized) static-string slice, if it exists.
+///
+/// Unlike `mimeTypeFromString`, this does NOT substitute canonical
+/// charset-appended forms (e.g. `text/plain` is returned as-is, not as
+/// `text/plain;charset=utf-8`). Use this where the WHATWG File/Blob API
+/// requires preserving the user-supplied MIME type verbatim.
+///
+/// Returns a slice into a static `_bytes` blob — safe to store without
+/// allocation tracking.
+pub fn mimeTypeInternedValue(this: *RareData, allocator: std.mem.Allocator, str: []const u8) ?[]const u8 {
+    if (this.mime_types == null) {
+        this.mime_types = bun.http.MimeType.createHashTable(
+            allocator,
+        ) catch |err| bun.handleOom(err);
+    }
+
+    if (this.mime_types.?.get(str)) |entry| {
+        return entry.slice();
+    }
+
+    return null;
+}
+
 pub const HotMap = struct {
     _map: bun.StringArrayHashMap(Entry),
 

src/bun.js/webcore/Blob.zig

@@ -1829,8 +1829,11 @@
                         }
                         blob.content_type_was_set = true;
 
-                        if (globalThis.bunVM().mimeType(slice)) |mime| {
-                            blob.content_type = mime.value;
+                        // WHATWG File API: the stored `type` must be the
+                        // lowercased input verbatim — do NOT canonicalize into
+                        // charset-appended forms like `text/plain;charset=utf-8`.
+                        if (globalThis.bunVM().mimeTypeInternedValue(slice)) |interned| {
+                            blob.content_type = interned;
                             break :inner;
                         }
                         const content_type_buf = bun.handleOom(allocator.alloc(u8, slice.len));
@@ -1929,8 +1932,10 @@
                             break :inner;
                         }
                         blob.content_type_was_set = true;
-                        if (vm.mimeType(str.slice())) |entry| {
-                            blob.content_type = entry.value;
+                        // WHATWG File API: preserve the lowercased input
+                        // verbatim; do not canonicalize to charset-appended forms.
+                        if (vm.mimeTypeInternedValue(slice)) |interned| {
+                            blob.content_type = interned;
                             break :inner;
                         }
                         const content_type_buf = bun.handleOom(allocator.alloc(u8, slice.len));
@@ -2323,11 +2328,14 @@
                 if (strings.isAllASCII(slice)) {
                     if (this.content_type_allocated) {
                         bun.default_allocator.free(this.content_type);
+                        this.content_type_allocated = false;
                     }
                     this.content_type_was_set = true;
 
-                    if (globalThis.bunVM().mimeType(slice)) |mime| {
-                        this.content_type = mime.value;
+                    // WHATWG File API: preserve the lowercased input
+                    // verbatim; do not canonicalize to charset-appended forms.
+                    if (globalThis.bunVM().mimeTypeInternedValue(slice)) |interned| {
+                        this.content_type = interned;
                     } else {
                         const content_type_buf = bun.handleOom(bun.default_allocator.alloc(u8, slice.len));
                         this.content_type = strings.copyLowercase(slice, content_type_buf);
@@ -2666,11 +2674,14 @@
                     if (strings.isAllASCII(slice)) {
                         if (this.content_type_allocated) {
                             bun.default_allocator.free(this.content_type);
+                            this.content_type_allocated = false;
                         }
                         this.content_type_was_set = true;
 
-                        if (globalThis.bunVM().mimeType(slice)) |mime| {
-                            this.content_type = mime.value;
+                        // WHATWG File API: preserve the lowercased input
+                        // verbatim; do not canonicalize to charset-appended forms.
+                        if (globalThis.bunVM().mimeTypeInternedValue(slice)) |interned| {
+                            this.content_type = interned;
                         } else {
                             const content_type_buf = bun.handleOom(bun.default_allocator.alloc(u8, slice.len));
                             this.content_type = strings.copyLowercase(slice, content_type_buf);
@@ -2923,14 +2934,16 @@
                 defer slicer.deinit();
                 const slice = slicer.slice();
                 if (!strings.isAllASCII(slice)) {
                     break :inner;
                 }
 
-                if (globalThis.bunVM().mimeType(slice)) |mime| {
-                    content_type = mime.value;
+                // WHATWG File API: preserve the lowercased input
+                // verbatim; do not canonicalize to charset-appended forms.
+                if (globalThis.bunVM().mimeTypeInternedValue(slice)) |interned| {
+                    content_type = interned;
                     break :inner;
                 }
 
                 content_type_was_allocated = slice.len > 0;
                 const content_type_buf = bun.handleOom(allocator.alloc(u8, slice.len));
                 content_type = strings.copyLowercase(slice, content_type_buf);
@@ -3354,8 +3367,10 @@
                                 }
                                 blob.content_type_was_set = true;
 
-                                if (globalThis.bunVM().mimeType(slice)) |mime| {
-                                    blob.content_type = mime.value;
+                                // WHATWG File API: preserve the lowercased input
+                                // verbatim; do not canonicalize to charset-appended forms.
+                                if (globalThis.bunVM().mimeTypeInternedValue(slice)) |interned| {
+                                    blob.content_type = interned;
                                     break :inner;
                                 }
                                 const content_type_buf = bun.handleOom(allocator.alloc(u8, slice.len));
@@ -3519,15 +3534,18 @@
 pub fn dupeWithContentType(this: *const Blob, include_content_type: bool) Blob {
     if (this.store != null) this.store.?.ref();
     var duped = this.*;
     duped.setNotHeapAllocated();
     if (duped.content_type_allocated and duped.isHeapAllocated() and !include_content_type) {
 
-        // for now, we just want to avoid a use-after-free here
-        if (jsc.VirtualMachine.get().mimeType(duped.content_type)) |mime| {
-            duped.content_type = mime.value;
+        // for now, we just want to avoid a use-after-free here.
+        // Use the interned value so the content_type isn't silently
+        // canonicalized into a charset-appended form (which would break
+        // WHATWG File API semantics).
+        if (jsc.VirtualMachine.get().mimeTypeInternedValue(duped.content_type)) |interned| {
+            duped.content_type = interned;
         } else {
             // TODO: fix this
             // this is a bug.
             // it means whenever
             duped.content_type = "";
         }

test/js/bun/util/inspect.test.js

@@ -587,24 +587,26 @@ describe("console.logging class displays names and extends", async () => {
 });
 
 it("console.log on a Blob shows name", () => {
+  // Per WHATWG File API, `text/plain` must NOT be canonicalized to
+  // `text/plain;charset=utf-8` — see #29257.
   const blob = new Blob(["foo"], { type: "text/plain" });
-  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  type: "text/plain;charset=utf-8"\n}');
+  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  type: "text/plain"\n}');
   blob.name = "bar";
-  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  name: "bar",\n  type: "text/plain;charset=utf-8"\n}');
+  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  name: "bar",\n  type: "text/plain"\n}');
   blob.name = "foobar";
-  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  name: "foobar",\n  type: "text/plain;charset=utf-8"\n}');
+  expect(Bun.inspect(blob)).toBe('Blob (3 bytes) {\n  name: "foobar",\n  type: "text/plain"\n}');
 
   const file = new File(["foo"], "bar.txt", { type: "text/plain" });
   expect(Bun.inspect(file)).toBe(
-    `File (3 bytes) {\n  name: "bar.txt",\n  type: "text/plain;charset=utf-8",\n  lastModified: ${file.lastModified}\n}`,
+    `File (3 bytes) {\n  name: "bar.txt",\n  type: "text/plain",\n  lastModified: ${file.lastModified}\n}`,
   );
   file.name = "foobar";
   expect(Bun.inspect(file)).toBe(
-    `File (3 bytes) {\n  name: "foobar",\n  type: "text/plain;charset=utf-8",\n  lastModified: ${file.lastModified}\n}`,
+    `File (3 bytes) {\n  name: "foobar",\n  type: "text/plain",\n  lastModified: ${file.lastModified}\n}`,
   );
   file.name = "";
   expect(Bun.inspect(file)).toBe(
-    `File (3 bytes) {\n  name: "",\n  type: "text/plain;charset=utf-8",\n  lastModified: ${file.lastModified}\n}`,
+    `File (3 bytes) {\n  name: "",\n  type: "text/plain",\n  lastModified: ${file.lastModified}\n}`,
   );
 });
 

test/js/web/structured-clone-blob-file.test.ts

@@ -15,7 +15,9 @@ describe("structuredClone with Blob and File", () => {
       const cloned = structuredClone(blob);
       expect(cloned).toBeInstanceOf(Blob);
       expect(cloned.size).toBe(11);
-      expect(cloned.type).toBe("text/plain;charset=utf-8");
+      // Per WHATWG File API, `text/plain` must NOT be canonicalized to
+      // `text/plain;charset=utf-8` — see #29257.
+      expect(cloned.type).toBe("text/plain");
 
       const originalText = await blob.text();
       const clonedText = await cloned.text();
@@ -63,11 +65,11 @@ describe("structuredClone with Blob and File", () => {
 
       expect(cloned.first).toBeInstanceOf(Blob);
       expect(cloned.first.size).toBe(5);
-      expect(cloned.first.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.first.type).toBe("text/plain");
 
       expect(cloned.second).toBeInstanceOf(Blob);
       expect(cloned.second.size).toBe(5);
-      expect(cloned.second.type).toBe("text/html;charset=utf-8");
+      expect(cloned.second.type).toBe("text/html");
     });
 
     test("deeply nested Blob", () => {
@@ -92,7 +94,7 @@ describe("structuredClone with Blob and File", () => {
       expect(cloned).toBeInstanceOf(File);
       expect(cloned.name).toBe("test.txt");
       expect(cloned.size).toBe(7);
-      expect(cloned.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.type).toBe("text/plain");
       expect(cloned.lastModified).toBe(1234567890000);
     });
 
@@ -103,7 +105,7 @@ describe("structuredClone with Blob and File", () => {
       expect(cloned).toBeInstanceOf(File);
       expect(cloned.name).toBe("test.txt");
       expect(cloned.size).toBe(7);
-      expect(cloned.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.type).toBe("text/plain");
       expect(cloned.lastModified).toBeGreaterThan(0);
     });
 
@@ -125,7 +127,7 @@ describe("structuredClone with Blob and File", () => {
       expect(cloned.file).toBeInstanceOf(File);
       expect(cloned.file.name).toBe("test.txt");
       expect(cloned.file.size).toBe(4);
-      expect(cloned.file.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.file.type).toBe("text/plain");
     });
 
     test("multiple Files in object", () => {
@@ -136,11 +138,11 @@ describe("structuredClone with Blob and File", () => {
 
       expect(cloned.txt).toBeInstanceOf(File);
       expect(cloned.txt.name).toBe("hello.txt");
-      expect(cloned.txt.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.txt.type).toBe("text/plain");
 
       expect(cloned.html).toBeInstanceOf(File);
       expect(cloned.html.name).toBe("world.html");
-      expect(cloned.html.type).toBe("text/html;charset=utf-8");
+      expect(cloned.html.type).toBe("text/html");
     });
   });
 
@@ -153,12 +155,12 @@ describe("structuredClone with Blob and File", () => {
 
       expect(cloned.blob).toBeInstanceOf(Blob);
       expect(cloned.blob.size).toBe(12);
-      expect(cloned.blob.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.blob.type).toBe("text/plain");
 
       expect(cloned.file).toBeInstanceOf(File);
       expect(cloned.file.name).toBe("test.txt");
       expect(cloned.file.size).toBe(12);
-      expect(cloned.file.type).toBe("text/plain;charset=utf-8");
+      expect(cloned.file.type).toBe("text/plain");
     });
 
     test("array with mixed Blob and File", () => {

test/regression/issue/29257.test.ts

@@ -0,0 +1,68 @@
+import { expect, test } from "bun:test";
+
+// https://github.com/oven-sh/bun/issues/29257
+//
+// Bun was rewriting `text/plain` (and `text/css`, `text/html`,
+// `application/json`, ...) to their charset-appended canonical forms
+// (`text/plain;charset=utf-8`, etc.) when the user set the `type` on a
+// Blob/File at construction time.
+//
+// Per the WHATWG File API (https://w3c.github.io/FileAPI/#blob), user
+// agents must NOT append a charset parameter to the media type.
+
+test("new File(..., { type: 'text/plain' }).type is preserved verbatim", () => {
+  const file = new File([], "empty.txt", { type: "text/plain" });
+  expect(file.type).toBe("text/plain");
+});
+
+test("new Blob([], { type: 'text/plain' }).type is preserved verbatim", () => {
+  const blob = new Blob([], { type: "text/plain" });
+  expect(blob.type).toBe("text/plain");
+});
+
+test("File/Blob type is preserved for other types Bun used to canonicalize", () => {
+  // These are the types Compact.toMimeType() substitutes into
+  // charset-appended forms for HTTP responses. None of them should leak
+  // the substitution into the File/Blob `type` property.
+  const types = [
+    "text/plain",
+    "text/css",
+    "text/html",
+    "text/javascript",
+    "application/json",
+    "application/javascript",
+  ];
+  for (const type of types) {
+    expect(new File([], "x", { type }).type).toBe(type);
+    expect(new Blob([], { type }).type).toBe(type);
+  }
+});
+
+test("File/Blob type with explicit charset is preserved verbatim", () => {
+  // A user who explicitly passes a charset parameter should get it back
+  // unchanged — not silently swapped for a different canonical form.
+  const file = new File([], "x.txt", { type: "text/plain;charset=utf-8" });
+  expect(file.type).toBe("text/plain;charset=utf-8");
+
+  const blob = new Blob([], { type: "text/plain;charset=utf-8" });
+  expect(blob.type).toBe("text/plain;charset=utf-8");
+});
+
+test("File/Blob type is lowercased (per WHATWG spec)", () => {
+  // The spec requires lowercasing but not charset canonicalization.
+  expect(new File([], "x", { type: "TEXT/PLAIN" }).type).toBe("text/plain");
+  expect(new Blob([], { type: "Text/Plain" }).type).toBe("text/plain");
+});
+
+test("uncommon MIME types still round-trip unchanged", () => {
+  // Types not in the interning table take the copyLowercase path. They
+  // should also round-trip verbatim (lowercased).
+  const file = new File([], "x", { type: "application/x-custom-type" });
+  expect(file.type).toBe("application/x-custom-type");
+});
+
+test("Bun.file(path, { type: 'text/plain' }).type is preserved verbatim", () => {
+  // Covers the `constructBunFile` path in Blob.zig.
+  const file = Bun.file(import.meta.path, { type: "text/plain" });
+  expect(file.type).toBe("text/plain");
+});