Fix for empty filename during edit

cps/web.py

@@ -3055,6 +3055,8 @@ def edit_book(book_id):
     # Check and handle Uploaded file
     if 'btn-upload-format' in request.files:
         requested_file = request.files['btn-upload-format']
+        # check for empty request
+        if requested_file.filename != '':
             if '.' in requested_file.filename:
                 file_ext = requested_file.filename.rsplit('.', 1)[-1].lower()
                 if file_ext not in ALLOWED_EXTENSIONS: