Download files into appropriate directories / set attachments mtimes

[birdie-github]

*.ogg -> audio
*.mp4 -> video
*.jpg *.png -> images

all the rest to documents or misc.

2. I'm not sure if Telegram API allows that, but please set downloaded files mtime to the time they were sent/received.

[nelo686]

I think that it would really nice if each channel/chat/supergroup downloaded files have its own directory.

[fabianonline]

The mtime feature is discussed in #81