Add multimedia (audio/video) files support

This adds support for common audio/video file formats: 
 * video: mp4, mpg, mpeg, mov, avi, wmv, flv, webm
 * audio: .aiff, .aif, .wma, .aac, .flac, .mp3, .ogg, .m4a

Most of them can be played with the standard player included in the browser. If they can't be played (unavailable codec e.g. `.wmv` on Firefox/Linux), an error message is displayed, but the video can still be downloaded (Right click -> `Save as...` or `Save link target as...`).

![minigal-format-error](https://cloud.githubusercontent.com/assets/3950390/4238829/109f60bc-39dc-11e4-99d4-4c72aceaab07.png)


There is still a minor cropping problem as shown below. The icon is resized to the full width/height of the box, but the text (filename) pushes it down, truncating the icon at the bottom.

![minigal-thumbs](https://cloud.githubusercontent.com/assets/3950390/4238778/b4e64fe2-39db-11e4-9b8b-c0c7239515d6.png)

Does this look OK to merge? (the cropping problem was already present with RAR, ZIP filetypes but not so obvious). I've used Nitrux icon theme to match previous icons but I can pick another theme where the problem is not so apparent, if you like.