[controller] FSAjaxController.upload: fixed empty files handling

vispa/controller/filesystem.py

@@ -320,7 +320,7 @@ class FSAjaxController(AbstractController):
 
                 while True:
                     data = part.file.read(1024 ** 2)
-                    if len(data) <= 0:
+                    if append and len(data) <= 0:
                         break
 
                     success, msg = fs.save_file_content(filename, data,