fix(blossom): send the real content type on upload

every upload went out as application/octet-stream because no caller passes
contentType, and a blossom server that sniffs the body rejects the
mismatch: uploading a valid png returned 400 'Content-Type header does not
match the file content, expected image/png'.

detect from the leading bytes rather than the file name. the bytes are
what the server checks, and an uploader should not be able to mislabel a
blob by renaming it. png, jpeg, gif, webp, pdf and svg, falling back to
octet-stream when nothing matches.

Author: TheCryptoDonkey

src/social/blossom.ts

@@ -18,6 +18,36 @@ export interface BlobDescriptor {
   uploaded?: number
 }
 
+// Blossom servers sniff the body and reject a mismatched Content-Type, so
+// `application/octet-stream` fails for any format they recognise. Detect from
+// the leading bytes rather than the file name: the bytes are what the server
+// checks, and an uploader should not be able to mislabel a blob by renaming it.
+const MAGIC: { type: string; test: (b: Uint8Array) => boolean }[] = [
+  { type: 'image/png', test: (b) => b[0] === 0x89 && b[1] === 0x50 && b[2] === 0x4e && b[3] === 0x47 },
+  { type: 'image/jpeg', test: (b) => b[0] === 0xff && b[1] === 0xd8 && b[2] === 0xff },
+  { type: 'image/gif', test: (b) => b[0] === 0x47 && b[1] === 0x49 && b[2] === 0x46 },
+  {
+    type: 'image/webp',
+    test: (b) =>
+      b[0] === 0x52 && b[1] === 0x49 && b[2] === 0x46 && b[3] === 0x46 &&
+      b[8] === 0x57 && b[9] === 0x45 && b[10] === 0x42 && b[11] === 0x50,
+  },
+  { type: 'application/pdf', test: (b) => b[0] === 0x25 && b[1] === 0x50 && b[2] === 0x44 && b[3] === 0x46 },
+  { type: 'image/svg+xml', test: (b) => Buffer.from(b.subarray(0, 256)).toString('utf8').includes('<svg') },
+]
+
+export function detectContentType(body: Uint8Array): string | undefined {
+  if (body.byteLength < 4) return undefined
+  for (const { type, test } of MAGIC) {
+    try {
+      if (test(body)) return type
+    } catch {
+      // a short or odd buffer simply does not match this signature
+    }
+  }
+  return undefined
+}
+
 /** Upload a file to a blossom media server */
 export async function handleBlossomUpload(
   ctx: SigningContext,
@@ -62,7 +92,7 @@ export async function handleBlossomUpload(
     method: 'PUT',
     headers: {
       Authorization: authHeader,
-      'Content-Type': args.contentType ?? 'application/octet-stream',
+      'Content-Type': args.contentType ?? detectContentType(body) ?? 'application/octet-stream',
     },
     body: Buffer.from(body),
     signal: AbortSignal.timeout(30_000),

test/social/blossom.test.ts

@@ -14,6 +14,7 @@ import {
   handleBlossomUsage,
   handleBlossomServersGet,
   handleBlossomServersSet,
+  detectContentType,
 } from '../../src/social/blossom.js'
 
 const TEST_NSEC = 'nsec1cxymst7yntfnvt4vkztk54q9muks6n77dn7qyhjpcvlxtkc6hy2s0364r8'
@@ -737,3 +738,26 @@ describe('blossom handlers', () => {
     })
   })
 })
+
+describe('detectContentType', () => {
+  it('recognises the formats a blossom server sniffs for', () => {
+    const png = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
+    const jpeg = new Uint8Array([0xff, 0xd8, 0xff, 0xe0])
+    const gif = new Uint8Array([0x47, 0x49, 0x46, 0x38, 0x39, 0x61])
+    const webp = new Uint8Array([
+      0x52, 0x49, 0x46, 0x46, 0, 0, 0, 0, 0x57, 0x45, 0x42, 0x50,
+    ])
+    const pdf = new Uint8Array([0x25, 0x50, 0x44, 0x46, 0x2d])
+    expect(detectContentType(png)).toBe('image/png')
+    expect(detectContentType(jpeg)).toBe('image/jpeg')
+    expect(detectContentType(gif)).toBe('image/gif')
+    expect(detectContentType(webp)).toBe('image/webp')
+    expect(detectContentType(pdf)).toBe('application/pdf')
+  })
+
+  it('falls back to undefined rather than guessing', () => {
+    expect(detectContentType(new Uint8Array([1, 2, 3, 4, 5]))).toBeUndefined()
+    expect(detectContentType(new Uint8Array([1, 2]))).toBeUndefined()
+    expect(detectContentType(new Uint8Array())).toBeUndefined()
+  })
+})