Mime and type of files are inconsistent

[clochix]

When uploading a file, we try to guess its MIME type and set a class (audio, video…). But we don't use the same method:

• MIME type is guessed from the file extension;
• class is determined by the content-type sent by the browser.

We should probably use the same method to set both of this value.

(sample : when uploading a .ogg file, Firefox set a wrong content-type of video/ogg, so the class of the document is set to video, but it's MIME type determined from the file extension is audio/ogg)

[clochix]

This issue is pretty annoying, as it prevent importing Ogg files to Music. They are labeled as Video, so Music doesn't import them.