Fix: possible memory leaks close #5277 (#5500)

### What problem does this PR solve?

close #5277 by make sure the file close

### Type of change

- [x] Performance Improvement

---------

Signed-off-by: yihong0618 <zouzou0208@gmail.com>

api/utils/file_utils.py

@@ -188,6 +188,7 @@ def thumbnail_img(filename, blob):
                 buffered = BytesIO()
             else:
                 break
+        pdf.close()
         return img
 
     elif re.match(r".*\.(jpg|jpeg|png|tif|gif|icon|ico|webp)$", filename):