Please check the following short examples of the issue:

Status: Downloaded newer image for python:3.7.4
Python 3.7.4 (default, Oct 17 2019, 05:59:21)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mimetypes
>>> str(mimetypes.guess_extension('image/bmp'))
'.bmp'


Status: Downloaded newer image for python:3.7.5
Python 3.7.5 (default, Nov 23 2019, 05:59:34)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mimetypes
>>> str(mimetypes.guess_extension('image/bmp'))
'None'
>>>

Status: Downloaded newer image for python:latest
Python 3.9.0 (default, Oct  6 2020, 21:52:53)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mimetypes
>>> str(mimetypes.guess_extension('image/bmp'))
'None'


I believe the issue is introduced here by duplicating the bmp key in types_map/_types_map_default

'.bmp'    : 'image/bmp'
https://github.com/python/cpython/pull/14375/files#diff-aed43839a49bace08b60186baa4b27ad69ecd6b61f928bd696b4fb670750774fR490

 '.bmp'    : 'image/x-ms-bmp',
https://github.com/python/cpython/pull/14375/files#diff-aed43839a49bace08b60186baa4b27ad69ecd6b61f928bd696b4fb670750774fR502