fix: When chatflow's file uploads changed from not being supported to… (#14341)

Co-authored-by: 刘江波 <jiangbo721@163.com>

api/models/workflow.py

             features["file_upload"]["enabled"] = image_enabled
             features["file_upload"]["number_limits"] = image_number_limits
             features["file_upload"]["allowed_file_upload_methods"] = image_transfer_methods
-            features["file_upload"]["allowed_file_types"] = ["image"]
+            features["file_upload"]["allowed_file_types"] = features["file_upload"].get("allowed_file_types", ["image"])
             features["file_upload"]["allowed_file_extensions"] = []
             del features["file_upload"]["image"]
             self._features = json.dumps(features)