fix: image files silently falling to text parser due to suffix dot mismatch

api_server.py passed "jpg" (no dot) from rsplit, but file_parser.py
parser dict keys all have dots (".jpg"), causing image files to fall
through to _parse_text() which fails on binary data, skipping ALL OCR
and layout analysis. Every image upload was affected.

- file_parser.py: normalize file_type to always have leading dot
- api_server.py: use Path.suffix instead of manual rsplit

backend/file_parser.py

@@ -41,7 +41,9 @@ def parse_file(file_path: str, file_type: str = "") -> dict:
     if not path.exists():
         return {"text": "", "file_type": file_type, "method": "none", "error": "文件不存在"}
 
-    suffix = file_type or path.suffix.lower()
+    suffix = path.suffix.lower()
+    if file_type:
+        suffix = file_type if file_type.startswith(".") else f".{file_type}"
 
     parsers = {
         ".png":  _parse_image,