compose: Include content-type in image-upload requests

Fixes: zulip#829

lib/widgets/compose_box.dart

@@ -4,6 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:image_picker/image_picker.dart';
+import 'package:mime/mime.dart';
 
 import '../api/exception.dart';
 import '../api/model/model.dart';
@@ -423,11 +424,17 @@ class _FixedDestinationContentInput extends StatelessWidget {
 /// A convenience class to represent data from the generic file picker,
 /// the media library, and the camera, in a single form.
 class _File {
-  _File({required this.content, required this.length, required this.filename});
+  _File({
+    required this.content,
+    required this.length,
+    required this.filename,
+    required this.mimeType,
+  });
 
   final Stream<List<int>> content;
   final int length;
   final String filename;
+  final String? mimeType;
 }
 
 Future<void> _uploadFiles({
@@ -474,14 +481,14 @@ Future<void> _uploadFiles({
   }
 
   for (final (tag, file) in uploadsInProgress) {
-    final _File(:content, :length, :filename) = file;
+    final _File(:content, :length, :filename, :mimeType) = file;
     Uri? url;
     try {
       final result = await uploadFile(store.connection,
         content: content,
         length: length,
         filename: filename,
-        mimeType: null, // TODO(#829)
+        mimeType: mimeType,
       );
       url = Uri.parse(result.uri);
     } catch (e) {
@@ -579,7 +586,17 @@ Future<Iterable<_File>> _getFilePickerFiles(BuildContext context, FileType type)
 
   return result.files.map((f) {
     assert(f.readStream != null);  // We passed `withReadStream: true` to pickFiles.
-    return _File(content: f.readStream!, length: f.size, filename: f.name);
+    String? mimeType;
+    if (f.path != null) {
+      mimeType = lookupMimeType(f.path!,
+        headerBytes: f.bytes?.take(defaultMagicNumbersMaxLength).toList());
+    }
+    return _File(
+      content: f.readStream!,
+      length: f.size,
+      filename: f.name,
+      mimeType: mimeType,
+    );
   });
 }
 
@@ -664,7 +681,14 @@ class _AttachFromCameraButton extends _AttachUploadsButton {
     }
     final length = await result.length();
 
-    return [_File(content: result.openRead(), length: length, filename: result.name)];
+    return [_File(
+      content: result.openRead(),
+      length: length,
+      filename: result.name,
+      mimeType: result.mimeType
+        // TODO pass headerBytes (what's a good way to get that?)
+        ?? lookupMimeType(result.path),
+    )];
   }
 }
 

test/widgets/compose_box_test.dart

@@ -281,6 +281,8 @@ void main() {
 
         testBinding.pickFilesResult = FilePickerResult([PlatformFile(
           readStream: Stream.fromIterable(['asdf'.codeUnits]),
+          // TODO test inference of mimeType from initial bytes, when
+          //   it can't be inferred from path
           path: '/private/var/mobile/Containers/Data/Application/foo/tmp/image.jpg',
           name: 'image.jpg',
           size: 12345,
@@ -304,6 +306,7 @@ void main() {
             ..field.equals('file')
             ..length.equals(12345)
             ..filename.equals('image.jpg')
+            ..contentType.asString.equals('image/jpeg')
             ..has<Future<List<int>>>((f) => f.finalize().toBytes(), 'contents')
               .completes((it) => it.deepEquals(['asdf'.codeUnits].expand((l) => l)))
           );
@@ -328,6 +331,8 @@ void main() {
         composeBoxController.topicController!.value = const TextEditingValue(text: 'some topic');
 
         testBinding.pickImageResult = XFile.fromData(
+          // TODO test inference of mimeType when it's missing here
+          mimeType: 'image/jpeg',
           utf8.encode('asdf'),
           name: 'image.jpg',
           length: 12345,
@@ -352,6 +357,7 @@ void main() {
             ..field.equals('file')
             ..length.equals(12345)
             ..filename.equals('image.jpg')
+            ..contentType.asString.equals('image/jpeg')
             ..has<Future<List<int>>>((f) => f.finalize().toBytes(), 'contents')
               .completes((it) => it.deepEquals(['asdf'.codeUnits].expand((l) => l)))
           );