Handle MIME types correctly

[pferreir]

We are currently sending Content-Type headers based on our own MIME type map. This is not a very good strategy, as Python includes the ​mimetypes module by default, and there's also ​python-magic that provides an extension-agnostic way (using libmagic) to guess the types.
The idea would be to replace our current map with some other mechanism based on an external lib (maybe one of these two). I would avoid trying to use the current interface methods, as it looks quite hacked, and it's easy to replace calls everywhere, they are not that many ;)