Strict mimetypes or not?

[frenzymadness]

We use mimetypes.guess_type in

kobo/kobo/http.py

Lines 32 to 40 in eaff9b4

	def get_content_type(self, file_name):
	"""Guess the mime type of a file.
	@param file_name: file name
	@type file_name: str
	@return: MIME type
	@rtype: str
	"""
	return mimetypes.guess_type(file_name)[0] or "application/octet-stream"

guess_type has strict option set to True by default which means that common_types included in Python stdlib are ignored.
For example, in a containerized environment without /etc/mime.types file, .rtf extension is not resolvable with strict mode turned on:

/usr/bin/python2.7
(None, None)
/usr/bin/python3.10
(None, None)
/usr/bin/python3.6
(None, None)
/usr/bin/python3.8
(None, None)
/usr/bin/python3.9
(None, None)

but without it, it works:

/usr/bin/python2.7
('application/rtf', None)
/usr/bin/python3.10
('application/rtf', None)
/usr/bin/python3.6
('application/rtf', None)
/usr/bin/python3.8
('application/rtf', None)
/usr/bin/python3.9
('application/rtf', None)

so turning off the strict mode might make it better for containerized environments and if we merge #162 we won't need mailcap package for tests (provides /etc/mime.types).