Concur with @sivert – the result of guess_extension() is non-deterministic between mimetypes module initialization.
$ python
Python 3.4.3 (default, Nov 17 2016, 01:08:31)
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpe
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpe
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpe
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpeg
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpeg
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpe
$ python -c 'import mimetypes;print(mimetypes.guess_extension("image/jpeg"))'
.jpg
$