fix: remove preview content-type override for aliyun oss compatibility

backend/server.js

@@ -686,30 +686,6 @@ function getBusyDownloadMessage() {
   return '当前网络繁忙，请稍后再试';
 }
 
-function getPreviewContentTypeByPath(filePath = '') {
-  const ext = path.extname(String(filePath || '')).toLowerCase();
-  const map = {
-    '.jpg': 'image/jpeg',
-    '.jpeg': 'image/jpeg',
-    '.png': 'image/png',
-    '.gif': 'image/gif',
-    '.bmp': 'image/bmp',
-    '.webp': 'image/webp',
-    '.svg': 'image/svg+xml',
-    '.mp4': 'video/mp4',
-    '.mov': 'video/quicktime',
-    '.webm': 'video/webm',
-    '.mkv': 'video/x-matroska',
-    '.mp3': 'audio/mpeg',
-    '.wav': 'audio/wav',
-    '.flac': 'audio/flac',
-    '.ogg': 'audio/ogg',
-    '.m4a': 'audio/mp4',
-    '.aac': 'audio/aac'
-  };
-  return map[ext] || '';
-}
-
 function parseDateTimeValue(value) {
   if (!value || typeof value !== 'string') {
     return null;
@@ -4645,12 +4621,6 @@ app.get('/api/files/download-url', authMiddleware, async (req, res) => {
         ? `inline; filename*=UTF-8''${encodeURIComponent(fileName)}`
         : `attachment; filename*=UTF-8''${encodeURIComponent(fileName)}`
     };
-    if (isPreviewMode) {
-      const previewContentType = getPreviewContentTypeByPath(fileName);
-      if (previewContentType) {
-        commandInput.ResponseContentType = previewContentType;
-      }
-    }
     const command = new GetObjectCommand(commandInput);
 
     // 生成签名 URL（1小时有效）