Keep the same naming scheme

[JuniorJPDJ]

Hi!
I'm trying to automate detection of new files posted on group I mounted.
The problem is your filenames are not consistent.

tgmount/tgmount/tgmount/dclasses.py

Lines 40 to 46 in b654d70

	def message_doc_filename_format(msg: Message, doc: TgmountDocument):
	attr_file_name = doc.atrributes.get('file_name')
	if attr_file_name:
	return ("%s %s" % (msg.id, attr_file_name)).encode()
	else:
	return ("msg_%s_doc" % msg.id).encode()

Could you please always prepend your names with ID, not make special case when name starts with msg and ID is somewhere inside? It will also fix sorting of files.

[JuniorJPDJ]

Guessing file extensions from mimetype provided by telegram when name is not known would be cool too ;)