Error in _valid_image_format

Closes #189 (closed)

dan/datasets/extract/__init__.py

@@ -39,7 +39,7 @@ def validate_char(char):
 def _valid_image_format(value: str):
     im_format = value
     if not im_format.startswith("."):
-        im_format.insert(0, ".")
+        im_format = "." + im_format
     return im_format