Change the method of get file extension. by myafer · Pull Request #475 · Baseflow/flutter_cache_manager

myafer · 2024-12-04T09:35:18Z

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix.

⤵️ What is the current behavior?

When the http response's Content-Type is not image and the _resizeimage is not work.

🆕 What is the new behavior (if this is a feature change)?

The new method is get files extension with the file's binary data.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

📝 Links to relevant issues/docs

🤔 Checklist before submitting

All projects build
Follows style guide lines (code style guide)
Relevant documentation was updated
Rebased onto current develop

1

cedvdb · 2025-04-22T13:00:46Z

@renefloor could this be reviewed ? using magic bytes is preferred over a potentially missing extension

settings.gradle and pubspec.yaml conflicted on lines main has since moved past (kotlin 2.3.20, build_runner ^2.14.0), so main's lines are kept. image_cache_manager.dart reformatted with dart format. Co-Authored-By: Claude <noreply@anthropic.com>

rickdijk · 2026-09-29T04:31:30Z

main has moved since you opened this, and our changes to example/android/settings.gradle and pubspec.yaml left it conflicting. I have merged main into your branch and resolved those conflicts. In both files main had already moved past the versions this pull request set (Kotlin 2.3.20 rather than 1.8.22, and build_runner ^2.14.0 rather than ^2.4.9), so I kept main's lines there. Your image_cache_manager.dart change is untouched apart from one line reflowed by dart format.

The resizer decided whether a file could be resized from the extension of the file on disk. That extension is derived from the url, so images served from extension-less urls, or from urls whose extension does not match the payload, were never resized. It also treated any zero-prefixed binary as an ico. - Detect jpeg, png, ico, cur and webp by their magic number, reading 12 bytes instead of 8 (webp needs the RIFF/WEBP marker at offset 8). - Keep an extension fallback for formats without a signature at the start of the file, like tga, which only stores a signature in its footer. - Read the header in a helper that always closes the file handle. - Restore the trailing newline in example/android/build.gradle, which was dropped by an earlier commit in this branch. - Bump to 3.4.6 with a dated changelog entry, as CONTRIBUTING now requires. Tests: cover an image served without an extension and a non-image payload served with an image extension.

myafer · 2026-10-19T03:28:40Z

Pushed a follow-up on top of your merge — thanks for resolving the settings.gradle / pubspec.yaml conflicts, I left your resolution untouched. The branch is still conflict-free against main.

What changed in image_cache_manager.dart:

Detect jpeg / png / ico / cur by magic number, and webp by its RIFF....WEBP marker, so 12 bytes are read instead of 8.
Fixed the swapped ICO (00 00 01 00) / CUR (00 00 02 00) values and dropped the 00 00 00 00 check, which matched any zero-prefixed binary.
Kept an extension fallback, but only for formats without a header signature (tga only writes one in its footer). supportedFileNames stays as it is.
Removed the debugPrint, and the header is now read in a helper that closes the handle in a finally.

Tests in test/image_cache_manager_test.dart: an image served from an extension-less url is resized, and a non-image payload served with a .png url is returned untouched.

Also bumped to 3.4.6 with a dated changelog entry per CONTRIBUTING, and restored the trailing newline in example/android/build.gradle that an earlier commit in this branch dropped.

dart format is clean. I could not run flutter test locally (mockito ^5.7.0 requires Dart 3.11, I only have 3.9), so it would be great if someone could approve the workflow run — it is still sitting at action_required.

Flutter decodes bmp and wbmp as well, so they are resizable too. Bmp has a 'BM' signature and can be detected from the content. Wbmp starts with a two byte header that is too generic to match on, so it joins tga in the extension fallback.

myafer · 2026-10-19T05:33:49Z

Follow-up: added bmp (BM) to the signatures, and wbmp to the extension fallback.

Wbmp is in Flutter's list of decodable formats, but it cannot be detected from the content: its header is 00 00, which matches any zero-prefixed binary — the same trap as the 00 00 00 00 ico check that was removed. So like tga it is only recognized by its extension.

Gif is intentionally left out: it is decodable, but _resizeImageFile re-encodes to PNG, so resizing would turn an animated gif into a still image. Happy to add it if that trade-off is acceptable.

Two bytes (the SOI marker) are enough to tell jpeg apart from the other formats that are checked, which is what SDWebImage and most detectors do. A file that is detected as an image but fails to decode then errors out of getImageFile, as it already did for a mislabeled extension before this change.

The extension that was passed to putFile came from the cached file path. For a url without an extension that path ends in the key itself, so the extension contained a slash and putFile failed to create the file. This is what the new test for an extension-less url caught. The resized image is always encoded as png anyway, so name it png instead of copying the extension of the url.

The resizer always re-encodes the image to png, which keeps a single frame. Now that webp is recognized by its signature, an animated webp would enter the resizer and silently lose its animation. That is the same reason gif is not recognized at all. Read the flags of the VP8X chunk, which is where an extended webp marks itself as animated, and pass those files through untouched. A static webp, with or without a VP8X chunk, is still resized.

A tga without an id field and without a colour map starts with the same four bytes as an ico or a cur, so the signatures cannot be told apart. It does not affect the result, since all of them are resizable, but it looks like a mistake without the note.

rickdijk

Thanks for picking this up. Detecting the format from the content is the right direction. A few things to look at before this can land; details are inline.

Two points that don't fit on a diff line:

shouldResize ignores maxHeight when maxWidth is set (existing bug, _resizeImageFile). The ternary parses as maxWidth != null ? image.width > maxWidth : (...), so the height check only runs when maxWidth is null. getImageFile(url, maxWidth: 200, maxHeight: 100) on a 150x400 image returns the 400px-tall original. It predates this PR, but since the function is being reworked anyway it would be good to fix it here:

final shouldResize = (maxWidth != null && image.width > maxWidth) ||
    (maxHeight != null && image.height > maxHeight);

PR description. Could you update it to the current PR template? The checklist is missing the 'Version bumped and dated CHANGELOG entry added' item and still mentions develop. main is the only branch now.

rickdijk · 2026-10-20T11:47:14Z

+const _imageSignatures = <List<int>>[
+  [0xFF, 0xD8], // jpeg
+  [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A], // png
+  [0x42, 0x4D], // bmp


The two-byte signatures (BM for bmp, FF D8 for jpeg) and 00 00 01/02 00 for ico/cur will also match files that aren't images. Such a file then goes into _decodeImage, and its error propagates out of getImageFile. Before, a file like that was returned untouched because its extension wasn't in the list. Whatever detection ends up being used, a decode failure should fall back to returning originalFile rather than erroring the stream.

rickdijk · 2026-10-20T11:47:14Z

+/// are resizable, and only the answer to "is this an image" is used.
+const _imageSignatures = <List<int>>[
+  [0xFF, 0xD8], // jpeg
+  [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A], // png


An animated PNG (APNG) matches this signature too, so it gets resized to a single static frame. That goes against the doc on _isImage saying animated images are passed through. Only webp has an animation check.

rickdijk · 2026-10-20T11:47:14Z

+/// file, so the only way to recognize them is by their extension. Tga only
+/// stores a signature in its footer, and wbmp starts with a two byte header
+/// (`00 00`) that is too generic to match on.
+const _extensionOnlyFormats = ['tga', 'wbmp'];


Flutter's image codecs can't decode TGA, so a .tga file (or a TGA that matches the ico/cur signature) will make _decodeImage throw instead of being resized. I'd drop tga here. The ico/cur comment above should also stop calling TGA resizable.

rickdijk · 2026-10-20T11:47:14Z

+  /// Images that would lose something when they are re-encoded as png, such as
+  /// an animated gif or an animated webp, are not considered an image here, so
+  /// they are passed through untouched.
+  Future<bool> _isImage(File file) async {


Rather than keeping a table of magic numbers plus a webp-only animation check, could we let the engine decide? For example, ui.ImmutableBuffer.fromUint8List + ui.ImageDescriptor.encoded, or ui.instantiateImageCodec: return the original when decoding fails or frameCount > 1, and resize otherwise. That covers every format the engine supports (including platform ones like HEIF) and every animated format (gif, APNG, animated webp), with no list to maintain.

rickdijk · 2026-10-20T11:47:14Z

      maxAge: maxAge,
-      fileExtension: fileExtension,
+      // The resized image is always encoded as png. The extension of the
+      // original file cannot be used: it is derived from the url, so it can


The stored file's extension doesn't come from the url. WebHelper._setDataFromHeaders builds the path from the response's Content-Type (response.fileExtension). The real failure case is a non-image Content-Type, e.g. application/octet-stream -> .bin. Could you fix this comment and the CHANGELOG wording to match? Storing as .png is correct, since the output is png-encoded.

rickdijk · 2026-10-20T11:47:14Z

@@ -7,6 +7,32 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_cache_manager/flutter_cache_manager.dart';

 const supportedFileNames = ['jpg', 'jpeg', 'png', 'tga', 'cur', 'ico'];


supportedFileNames is exported publicly, but nothing uses it anymore and it no longer reflects what gets resized (no webp or bmp). Removing it would be breaking, so maybe mark it @Deprecated with a note that detection is now content-based?

rickdijk · 2026-10-20T11:47:15Z

    });
  });
+
+  group('Test image detection', () {


Could you add tests for the new formats too: resizing a static webp and a bmp, checking that the resized file is stored with a .png extension, and that a file that fails to decode is returned untouched? Also note that the 'without file extension' case sets the file name directly, so it doesn't go through the Content-Type path where the real issue occurs.

myafer added 9 commits December 4, 2024 17:19

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4e03901

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Update file_service_compat.dart

74e71ca

Update image_cache_manager.dart

b6105f7

fix: fix method name

6028618

Update build.gradle

4ec2651

Update build.gradle

b8ebe93

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Update build.gradle

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Update build.gradle

1a55b01

rickdijk changed the base branch from develop to main September 16, 2026 00:24

Merge main into develop

b0ed040

settings.gradle and pubspec.yaml conflicted on lines main has since moved past (kotlin 2.3.20, build_runner ^2.14.0), so main's lines are kept. image_cache_manager.dart reformatted with dart format. Co-Authored-By: Claude <noreply@anthropic.com>

myafer force-pushed the develop branch from 4784d84 to a7dac2a Compare October 19, 2026 05:43

myafer added 3 commits October 19, 2026 13:51

rickdijk reviewed Oct 20, 2026

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Change the method of get file extension. - #475

Change the method of get file extension.#475
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		@@ -7,6 +7,32 @@ import 'package:flutter/widgets.dart';
		import 'package:flutter_cache_manager/flutter_cache_manager.dart';

		const supportedFileNames = ['jpg', 'jpeg', 'png', 'tga', 'cur', 'ico'];

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✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

⤵️ What is the current behavior?

🆕 What is the new behavior (if this is a feature change)?

💥 Does this PR introduce a breaking change?

🐛 Recommendations for testing

📝 Links to relevant issues/docs

🤔 Checklist before submitting

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