fix preview url (#26059)

api/fields/file_fields.py

@@ -32,7 +32,8 @@ file_fields = {
     "mime_type": fields.String,
     "created_by": fields.String,
     "created_at": TimestampField,
-    "preview_url": fields.String(attribute="source_url"),
+    "preview_url": fields.String,
+    "source_url": fields.String,
 }