diff --git a/src/codecs/browser-jpeg/decoder.ts b/src/codecs/browser-jpeg/decoder.ts
index d6107bdd..6321d3a1 100644
--- a/src/codecs/browser-jpeg/decoder.ts
+++ b/src/codecs/browser-jpeg/decoder.ts
@@ -1,7 +1,6 @@
 import { canDecodeImage, fileToBitmap } from '../../lib/util';
 
 export const name = 'Browser JPEG Decoder';
-export const supportedExtensions = ['jpg', 'jpeg'];
 export const supportedMimeTypes = ['image/jpeg'];
 export async function decode(file: File): Promise<ImageBitmap> {
   return fileToBitmap(file);
@@ -13,3 +12,7 @@ const jpegFile = 'data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCAgI
 export function isSupported(): Promise<boolean> {
   return canDecodeImage(jpegFile);
 }
+
+export function canHandleMimeType(mimeType: string): boolean {
+  return supportedMimeTypes.includes(mimeType);
+}
diff --git a/src/codecs/browser-png/decoder.ts b/src/codecs/browser-png/decoder.ts
index 357b0ab9..47ecd6b8 100644
--- a/src/codecs/browser-png/decoder.ts
+++ b/src/codecs/browser-png/decoder.ts
@@ -1,7 +1,6 @@
 import { canDecodeImage, fileToBitmap } from '../../lib/util';
 
 export const name = 'Browser PNG Decoder';
-export const supportedExtensions = ['png'];
 export const supportedMimeTypes = ['image/png'];
 export async function decode(file: File): Promise<ImageBitmap> {
   return fileToBitmap(file);
@@ -13,3 +12,7 @@ const pngFile = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfF
 export function isSupported(): Promise<boolean> {
   return canDecodeImage(pngFile);
 }
+
+export function canHandleMimeType(mimeType: string): boolean {
+  return supportedMimeTypes.includes(mimeType);
+}
diff --git a/src/codecs/browser-webp/decoder.ts b/src/codecs/browser-webp/decoder.ts
index baa962fd..9a570685 100644
--- a/src/codecs/browser-webp/decoder.ts
+++ b/src/codecs/browser-webp/decoder.ts
@@ -1,7 +1,6 @@
 import { canDecodeImage, fileToBitmap } from '../../lib/util';
 
 export const name = 'Browser WebP Decoder';
-export const supportedExtensions = ['webp'];
 export const supportedMimeTypes = ['image/webp'];
 export async function decode(file: File): Promise<ImageBitmap> {
   return fileToBitmap(file);
@@ -13,3 +12,7 @@ const webpFile = 'data:image/webp;base64,UklGRh4AAABXRUJQVlA4TBEAAAAvAAAAAAfQ//7
 export function isSupported(): Promise<boolean> {
   return canDecodeImage(webpFile);
 }
+
+export function canHandleMimeType(mimeType: string): boolean {
+  return supportedMimeTypes.includes(mimeType);
+}
diff --git a/src/codecs/decoders.ts b/src/codecs/decoders.ts
index 44c0b977..b49a2495 100644
--- a/src/codecs/decoders.ts
+++ b/src/codecs/decoders.ts
@@ -3,12 +3,13 @@ import * as browserPNG from './browser-png/decoder';
 import * as browserWebP from './browser-webp/decoder';
 import * as wasmWebP from './webp/decoder';
 
+import { sniffMimeType } from '../lib/util';
+
 export interface Decoder {
   name: string;
   decode(file: File): Promise<ImageBitmap>;
   isSupported(): Promise<boolean>;
-  supportedMimeTypes: string[];
-  supportedExtensions: string[];
+  canHandleMimeType(mimeType: string): boolean;
 }
 
 // We load all decoders and filter out the unsupported ones.
@@ -19,9 +20,9 @@ export const decodersPromise: Promise<Decoder[]> = Promise.all(
     wasmWebP,
     browserWebP,
   ]
-  .map(async (encoder) => {
-    if (await encoder.isSupported()) {
-      return encoder;
+  .map(async (decoder) => {
+    if (await decoder.isSupported()) {
+      return decoder;
     }
     return null;
   }),
@@ -31,12 +32,9 @@ export const decodersPromise: Promise<Decoder[]> = Promise.all(
 
 export async function findDecoder(file: File): Promise<Decoder | undefined> {
   const decoders = await decodersPromise;
-  // Prefer a match on mime type over a match on file extension
-  const decoder = decoders.find(decoder => decoder.supportedMimeTypes.includes(file.type));
-  if (decoder) {
-    return decoder;
+  const mimeType = await sniffMimeType(file);
+  if (!mimeType) {
+    return;
   }
-  return decoders.find(decoder =>
-    decoder.supportedExtensions.some(extension =>
-       file.name.endsWith(`.${extension}`)));
+  return decoders.find(decoder => decoder.canHandleMimeType(mimeType));
 }
diff --git a/src/codecs/webp/decoder.ts b/src/codecs/webp/decoder.ts
index 685a2d7e..497d3eab 100644
--- a/src/codecs/webp/decoder.ts
+++ b/src/codecs/webp/decoder.ts
@@ -2,7 +2,6 @@ import { blobToArrayBuffer, imageDataToBitmap } from '../../lib/util';
 import DecoderWorker  from './Decoder.worker';
 
 export const name = 'WASM WebP Decoder';
-export const supportedExtensions = ['webp'];
 export const supportedMimeTypes = ['image/webp'];
 export async function decode(file: File): Promise<ImageBitmap> {
   const decoder = await new DecoderWorker();
@@ -13,3 +12,7 @@ export async function decode(file: File): Promise<ImageBitmap> {
 export async function isSupported(): Promise<boolean> {
   return true;
 }
+
+export function canHandleMimeType(mimeType: string): boolean {
+  return supportedMimeTypes.includes(mimeType);
+}
diff --git a/src/lib/util.ts b/src/lib/util.ts
index 1c731b09..1a35eb02 100644
--- a/src/lib/util.ts
+++ b/src/lib/util.ts
@@ -118,3 +118,28 @@ export function blobToArrayBuffer(blob: Blob): Promise<ArrayBuffer> {
     fileReader.readAsArrayBuffer(blob);
   });
 }
+
+const magicNumberToMimeType = new Map<RegExp, string>([
+  [/^%PDF-/, 'application/pdf'],
+  [/^GIF87a/, 'image/gif'],
+  [/^GIF89a/, 'image/gif'],
+  [/^\x89PNG\x0D\x0A\x1A\x0A/, 'image/png'],
+  [/^\xFF\xD8\xFF/, 'image/jpeg'],
+  [/^BM/, 'image/bmp'],
+  [/^I I/, 'image/tiff'],
+  [/^II*/, 'image/tiff'],
+  [/^MM\x00*/, 'image/tiff'],
+  [/^RIFF....WEBPVP8 /, 'image/webp'],
+]);
+
+export async function sniffMimeType(blob: Blob): Promise<string | undefined> {
+  const firstChunk = await blobToArrayBuffer(blob.slice(0, 1024));
+  const firstChunkString = Array.from(
+    new Uint8Array(firstChunk)).map(v => String.fromCodePoint(v),
+  ).join('');
+  for (const [detector, mimeType] of magicNumberToMimeType.entries()) {
+    if (detector.test(firstChunkString)) {
+      return mimeType;
+    }
+  }
+}