The mimetype builtin module allows users to guess extension for a given mimetype through the `mimetypes.guess_extension` function.

Default mimetypes are stored in `types_map` and `_types_map_default` dictionaries that maps extensions to mimetypes. Those dictionaries are created by `_default_mime_types` function in `cpython/Lib/mimetypes.py`.

If a given extension have more than one mimetype, this information is lost.
This happens currently for ".bmp" extension in CPython's codebase.

This can be seen in the linked code below:
https://github.com/python/cpython/blob/110a47c4f42cf4db88edc1876899fff8f05190fb/Lib/mimetypes.py#L490-L502

Here is an example in an interactive IPython session:
```
In [1]: import mimetypes

In [2]: mimetypes.guess_extension('image/bmp')
Out[2]: '.bmp'

In [3]: mimetypes.guess_extension('image/x-ms-bmp')

In [4]:
```

The issue has been found by using Semmle's LGTM: https://lgtm.com/projects/g/python/cpython/snapshot/d099f261c762ac81042e47b530d279f932d89e09/files/Lib/mimetypes.py?sort=name&dir=ASC&mode=heatmap


PS / offtopic / loud thinking: Maybe there should be a debug build of CPython that would detect such key overwrites during dicts initialisation and warn about them?

To be more specific and to keep this information historically, the .bmp registers two mimetypes - 'image/bmp' and 'image/x-ms-bmp'.

Below a part of the relevant code.
```
    types_map = _types_map_default = {
        # (...)
        '.bmp'    : 'image/bmp',
        '.gif'    : 'image/gif',
        '.ief'    : 'image/ief',
        '.jpg'    : 'image/jpeg',
        '.jpe'    : 'image/jpeg',
        '.jpeg'   : 'image/jpeg',
        '.png'    : 'image/png',
        '.svg'    : 'image/svg+xml',
        '.tiff'   : 'image/tiff',
        '.tif'    : 'image/tiff',
        '.ico'    : 'image/vnd.microsoft.icon',
        '.ras'    : 'image/x-cmu-raster',
        '.bmp'    : 'image/x-ms-bmp',
```

This appears to have been fixed by issue #4963 and backported to the 3.7 and 3.8 branches:

Python 3.7.4+ (heads/3.7-dirty:e7bec26937, Jul 16 2019, 12:53:26) 
[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/bmp')
'.bmp'
>>> mimetypes.guess_extension('image/x-ms-bmp')
'.bmp'

It doesn't seem so... In Python 3.8.5 this code is still seen in mimetypes.py (line 492-504):

        '.bmp'    : 'image/bmp',
        '.gif'    : 'image/gif',
        '.ief'    : 'image/ief',
        '.jpg'    : 'image/jpeg',
        '.jpe'    : 'image/jpeg',
        '.jpeg'   : 'image/jpeg',
        '.png'    : 'image/png',
        '.svg'    : 'image/svg+xml',
        '.tiff'   : 'image/tiff',
        '.tif'    : 'image/tiff',
        '.ico'    : 'image/vnd.microsoft.icon',
        '.ras'    : 'image/x-cmu-raster',
        '.bmp'    : 'image/x-ms-bmp',

This was fixed in https://github.com/python/cpython/pull/26300 but wasn't backported.

New changeset d9db07a3100105768ba83ffd67991e78452bb22e by andrei kulakov in branch 'main':
bpo-37529: Add test for guessing extensions (GH-28243)
https://github.com/python/cpython/commit/d9db07a3100105768ba83ffd67991e78452bb22e