fix: Proxy file downloads through Admin UI for mTLS support

The DownloadFile function previously used browser redirect, which would
fail when filer requires mutual TLS (client certificates) since the
browser doesn't have these certificates.

Now the Admin UI server proxies the download, using its TLS-aware HTTP
client with the configured client certificates, then streams the
response to the browser.

weed/admin/handlers/file_browser_handlers.go

@@ -470,7 +470,8 @@ func (h *FileBrowserHandlers) validateAndCleanFilePath(filePath string) (string,
 	return cleanPath, nil
 }
 
-// DownloadFile handles file download requests
+// DownloadFile handles file download requests by proxying through the Admin UI server
+// This ensures mTLS works correctly since the Admin UI server has the client certificates
 func (h *FileBrowserHandlers) DownloadFile(c *gin.Context) {
 	filePath := c.Query("path")
 	if filePath == "" {
@@ -485,6 +486,12 @@ func (h *FileBrowserHandlers) DownloadFile(c *gin.Context) {
 		return
 	}
 
+	// Validate filer address to prevent SSRF
+	if err := h.validateFilerAddress(filerAddress); err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "Invalid filer address configuration"})
+		return
+	}
+
 	// Validate and sanitize the file path
 	cleanFilePath, err := h.validateAndCleanFilePath(filePath)
 	if err != nil {
@@ -500,13 +507,48 @@ func (h *FileBrowserHandlers) DownloadFile(c *gin.Context) {
 		return
 	}
 
+	// Proxy the download through the Admin UI server to support mTLS
+	// lgtm[go/ssrf]
+	// Safe: filerAddress validated by validateFilerAddress() to match configured filer
+	// Safe: cleanFilePath validated and cleaned by validateAndCleanFilePath() to prevent path traversal
+	clientWithTimeout := http.Client{
+		Transport: h.httpClient.Client.Transport,
+		Timeout:   5 * time.Minute, // Longer timeout for large file downloads
+	}
+	resp, err := clientWithTimeout.Get(downloadURL)
+	if err != nil {
+		c.JSON(http.StatusBadGateway, gin.H{"error": "Failed to fetch file from filer: " + err.Error()})
+		return
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		c.JSON(resp.StatusCode, gin.H{"error": fmt.Sprintf("Filer returned status %d", resp.StatusCode)})
+		return
+	}
+
 	// Set headers for file download
 	fileName := filepath.Base(cleanFilePath)
 	c.Header("Content-Disposition", fmt.Sprintf("attachment; filename=\"%s\"", fileName))
-	c.Header("Content-Type", "application/octet-stream")
 
-	// Proxy the request to filer
-	c.Redirect(http.StatusFound, downloadURL)
+	// Use content type from filer response, or default to octet-stream
+	contentType := resp.Header.Get("Content-Type")
+	if contentType == "" {
+		contentType = "application/octet-stream"
+	}
+	c.Header("Content-Type", contentType)
+
+	// Set content length if available
+	if resp.ContentLength > 0 {
+		c.Header("Content-Length", fmt.Sprintf("%d", resp.ContentLength))
+	}
+
+	// Stream the response body to the client
+	c.Status(http.StatusOK)
+	_, err = io.Copy(c.Writer, resp.Body)
+	if err != nil {
+		glog.Errorf("Error streaming file download: %v", err)
+	}
 }
 
 // ViewFile handles file viewing requests (for text files, images, etc.)