From 2a4e573dcdf5455ebdf0a2b3541e190021a649f1 Mon Sep 17 00:00:00 2001
From: Sergey Fedotov <sergey89@gmail.com>
Date: Wed, 8 Dec 2021 23:29:07 +0300
Subject: [PATCH] Serve assets with correct content-type header value (#94357)

---
 .../lib/src/isolated/devfs_web.dart           | 11 +++----
 .../general.shard/web/devfs_web_test.dart     | 33 +++++++++----------
 2 files changed, 20 insertions(+), 24 deletions(-)

diff --git a/packages/flutter_tools/lib/src/isolated/devfs_web.dart b/packages/flutter_tools/lib/src/isolated/devfs_web.dart
index 1d5b3d4a17d..e9fd1d07b68 100644
--- a/packages/flutter_tools/lib/src/isolated/devfs_web.dart
+++ b/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;
diff --git a/packages/flutter_tools/test/general.shard/web/devfs_web_test.dart b/packages/flutter_tools/test/general.shard/web/devfs_web_test.dart
index 9dfd5aa3cf9..b9b4e192ad3 100644
--- a/packages/flutter_tools/test/general.shard/web/devfs_web_test.dart
+++ b/packages/flutter_tools/test/general.shard/web/devfs_web_test.dart
@@ -539,7 +539,21 @@ 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 {
+  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/foo.js')));
+
+    expect(response.headers, allOf(<Matcher>[
+      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', () => 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));
@@ -554,21 +568,6 @@ 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(<int>[1, 2, 3]);
-
-    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.contentTypeHeader, 'application/octet-stream'),
-    ]));
-    expect((await response.read().toList()).first, source.readAsBytesSync());
-  }));
-
   test('serves valid etag header for asset files with non-ascii characters', () => testbed.run(() async {
     globals.fs.file(globals.fs.path.join('build', 'flutter_assets', 'fooπ'))
       ..createSync(recursive: true)
@@ -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());
   }));