Serve assets with correct content-type header value (#94357)

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -455,14 +455,11 @@ class WebAssetServer implements AssetReader {
     // Attempt to determine the file's mime type. if this is not provided some
     // browsers will refuse to render images/show video etc. If the tool
     // cannot determine a mime type, fall back to application/octet-stream.
-    String mimeType;
-    if (length >= 12) {
-      mimeType = mime.lookupMimeType(
+    final String mimeType = mime.lookupMimeType(
         file.path,
-        headerBytes: await file.openRead(0, 12).first,
-      );
-    }
-    mimeType ??= _kDefaultMimeType;
+        headerBytes: await file.openRead(0, mime.defaultMagicNumbersMaxLength).first,
+    ) ?? _kDefaultMimeType;
+
     headers[HttpHeaders.contentLengthHeader] = length.toString();
     headers[HttpHeaders.contentTypeHeader] = mimeType;
     headers[HttpHeaders.etagHeader] = etag;

packages/flutter_tools/test/general.shard/web/devfs_web_test.dart

@@ -539,31 +539,30 @@ void main() {
     expect((await response.read().toList()).first, source.readAsBytesSync());
   }));
 
-  test('serves asset files files from in filesystem with unknown mime type and length > 12', () => testbed.run(() async {
-    final File source = globals.fs.file(globals.fs.path.join('build', 'flutter_assets', 'foo'))
-      ..createSync(recursive: true)
-      ..writeAsBytesSync(List<int>.filled(100, 0));
+  test('serves asset files from in filesystem with known mime type and empty content', () => testbed.run(() async {
+    final File source = globals.fs.file(globals.fs.path.join('web', 'foo.js'))
+      ..createSync(recursive: true);
 
     final Response response = await webAssetServer
-      .handleRequest(Request('GET', Uri.parse('http://foobar/assets/foo')));
+        .handleRequest(Request('GET', Uri.parse('http://foobar/foo.js')));
 
     expect(response.headers, allOf(<Matcher>[
-      containsPair(HttpHeaders.contentLengthHeader, '100'),
-      containsPair(HttpHeaders.contentTypeHeader, 'application/octet-stream'),
+      containsPair(HttpHeaders.contentLengthHeader, '0'),
+      containsPair(HttpHeaders.contentTypeHeader, 'application/javascript'),
     ]));
     expect((await response.read().toList()).first, source.readAsBytesSync());
   }));
 
-  test('serves asset files files from in filesystem with unknown mime type and length < 12', () => testbed.run(() async {
+  test('serves asset files files from in filesystem with unknown mime type', () => testbed.run(() async {
     final File source = globals.fs.file(globals.fs.path.join('build', 'flutter_assets', 'foo'))
       ..createSync(recursive: true)
-      ..writeAsBytesSync(<int>[1, 2, 3]);
+      ..writeAsBytesSync(List<int>.filled(100, 0));
 
     final Response response = await webAssetServer
       .handleRequest(Request('GET', Uri.parse('http://foobar/assets/foo')));
 
     expect(response.headers, allOf(<Matcher>[
-      containsPair(HttpHeaders.contentLengthHeader, '3'),
+      containsPair(HttpHeaders.contentLengthHeader, '100'),
       containsPair(HttpHeaders.contentTypeHeader, 'application/octet-stream'),
     ]));
     expect((await response.read().toList()).first, source.readAsBytesSync());
@@ -594,7 +593,7 @@ void main() {
 
     expect(response.headers, allOf(<Matcher>[
       containsPair(HttpHeaders.contentLengthHeader, '3'),
-      containsPair(HttpHeaders.contentTypeHeader, 'application/octet-stream'),
+      containsPair(HttpHeaders.contentTypeHeader, 'text/x-dart'),
     ]));
     expect((await response.read().toList()).first, source.readAsBytesSync());
   }));