Fix misleading documentation

flask_admin/form/upload.py

@@ -152,7 +152,7 @@ class FileUploadField(fields.StringField):
                         upload = FileUploadField('File', namegen=prefix_name)
 
             :param allowed_extensions:
-                List of allowed extensions. If not provided, will allow any file.
+                List of allowed extensions. If not provided, then gif, jpg, jpeg, png and tiff will be allowed.
         """
         self.base_path = base_path
         self.relative_path = relative_path