fix(upload): set explicit MIME type on multipart upload

Without a content type header, http package defaults to
application/octet-stream which the server rejects. Derive MIME
from file extension using a lookup map (Dart 2 compatible).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/api/api_client.dart

@@ -2,6 +2,7 @@
 import 'dart:convert';
 import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
+import 'package:http_parser/http_parser.dart';
 import '../storage/token_storage.dart';
 
 class ApiClient {
@@ -105,7 +106,21 @@ class ApiClient {
   /// `{ fileId, url, name, type, mimeType, size, backend }`
   Future<Map<String, dynamic>> uploadFile(String url, String filePath) async {
     final request = http.MultipartRequest('POST', Uri.parse(url));
-    request.files.add(await http.MultipartFile.fromPath('file', filePath));
+    const _mimeMap = <String, List<String>>{
+      'jpg':  ['image', 'jpeg'],
+      'jpeg': ['image', 'jpeg'],
+      'png':  ['image', 'png'],
+      'webp': ['image', 'webp'],
+      'mp4':  ['video', 'mp4'],
+      'mov':  ['video', 'quicktime'],
+    };
+    final ext = filePath.split('.').last.toLowerCase();
+    final parts = _mimeMap[ext] ?? ['image', 'jpeg'];
+    request.files.add(await http.MultipartFile.fromPath(
+      'file',
+      filePath,
+      contentType: MediaType(parts[0], parts[1]),
+    ));
 
     late http.StreamedResponse streamed;
     try {