fix: use preview-mode signed URLs and graceful media preview fallback

backend/server.js

@@ -686,6 +686,30 @@ 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;
@@ -4534,6 +4558,8 @@ app.post('/api/files/upload-complete', authMiddleware, async (req, res) => {
 // 生成 OSS 下载签名 URL（用户直连 OSS 下载，不经过后端）
 app.get('/api/files/download-url', authMiddleware, async (req, res) => {
   const filePath = req.query.path;
+  const mode = String(req.query.mode || 'download').toLowerCase();
+  const isPreviewMode = mode === 'preview';
 
   if (!filePath) {
     return res.status(400).json({
@@ -4610,12 +4636,22 @@ app.get('/api/files/download-url', authMiddleware, async (req, res) => {
       }
     }
 
-    // 创建 GetObject 命令
-    const command = new GetObjectCommand({
+    // 创建 GetObject 命令（预览模式使用 inline，下载模式使用 attachment）
+    const fileName = normalizedPath.split('/').pop() || 'download.bin';
+    const commandInput = {
       Bucket: bucket,
       Key: objectKey,
-      ResponseContentDisposition: `attachment; filename="${encodeURIComponent(normalizedPath.split('/').pop())}"`
-    });
+      ResponseContentDisposition: isPreviewMode
+        ? `inline; filename*=UTF-8''${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）
     const signedUrl = await getSignedUrl(client, command, { expiresIn: 3600 });