Preserve MIME type in toFile

src/internal/to-file.ts

@@ -95,12 +95,19 @@ export async function toFile(
     if (value instanceof File) {
       return value;
     }
-    return makeFile([await value.arrayBuffer()], value.name);
+    return makeFile([await value.arrayBuffer()], value.name, {
+      type: value.type,
+      lastModified: value.lastModified,
+      ...options,
+    });
   }
 
   if (isResponseLike(value)) {
     const blob = await value.blob();
     name ||= new URL(value.url).pathname.split(/[\\/]/).pop();
+    if (!options?.type && blob.type) {
+      options = { ...options, type: blob.type };
+    }
 
     return makeFile(await getBytes(blob), name, options);
   }
@@ -110,7 +117,14 @@ export async function toFile(
   name ||= getName(value);
 
   if (!options?.type) {
-    const type = parts.find((part) => typeof part === 'object' && 'type' in part && part.type);
+    const type = parts.find(
+      (part): part is Blob =>
+        typeof part === 'object' &&
+        part !== null &&
+        'type' in part &&
+        typeof part.type === 'string' &&
+        part.type.length > 0,
+    )?.type;
     if (typeof type === 'string') {
       options = { ...options, type };
     }

tests/uploads.test.ts

@@ -43,6 +43,15 @@ describe('toFile', () => {
     expect(file.name).toEqual('audio.mp3');
   });
 
+  it('preserves the MIME type from a Response', async () => {
+    const response = mockResponse({
+      url: 'https://example.com/my/audio.mp3',
+      content: new Blob(['foo'], { type: 'audio/mpeg' }),
+    });
+    const file = await toFile(response);
+    expect(file.type).toEqual('audio/mpeg');
+  });
+
   it('extracts a file name from a File', async () => {
     const input = new File(['foo'], 'input.jsonl');
     const file = await toFile(input);
@@ -55,6 +64,27 @@ describe('toFile', () => {
     expect(file.name).toEqual('uploads.test.ts');
   });
 
+  it('preserves metadata from a File-like object', async () => {
+    const input = {
+      name: 'input.jsonl',
+      type: 'application/jsonl',
+      size: 3,
+      lastModified: 123,
+      text: async () => 'foo',
+      slice: () => new Blob(['foo'], { type: 'application/jsonl' }),
+      arrayBuffer: async () => new Blob(['foo']).arrayBuffer(),
+    };
+    const file = await toFile(input);
+    expect(file.name).toEqual('input.jsonl');
+    expect(file.type).toEqual('application/jsonl');
+    expect(file.lastModified).toEqual(123);
+  });
+
+  it('infers the MIME type from Blob parts', async () => {
+    const file = await toFile(new Blob(['foo'], { type: 'text/plain' }), 'input.txt');
+    expect(file.type).toEqual('text/plain');
+  });
+
   it('does not copy File objects', async () => {
     const input = new File(['foo'], 'input.jsonl', { type: 'jsonl' });
     const file = await toFile(input);