I think this one’s quite easy to reproduce:

  Python 3.7.4 (default, Jul 11 2019, 01:08:00) 
  [Clang 10.0.1 (clang-1001.0.46.4)] on darwin
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import mimetypes
  >>> mimetypes.guess_extension("image/jpg")  # Expected ".jpg"
  >>> mimetypes.guess_extension("image/jpeg")  # Expected ".jpg"
  '.jpe'

According to MDN

  https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Complete_list_of_MIME_types

only "image/jpeg" is a valid MIME type; however, I’ve seen quite a bit of "image/jpg" out in the wild and I think that ought to be accounted for too.

Before I look into submitting a PR I wanted to confirm that this is an issue that ought to be fixed. I think it is.