mimetypes: add audio/video support for apple airplay

mimetypes: add audio/video support for apple airplay

Airplay of a statically hosted video file from one Apple device
to another fails due to unrecognized content-type.

Let's assume we have url1: http://openwrt.local/luci-static/video.mp4.
If url1 is passed to a HTML5 browser, it will download the file instead
of playing it back as it thinks it is a binary (no entry for mp4
extension atm). Let's also assume we worked this around by wrapping url1
in HTML5 video tag, inside of an html file which we will put at
url2: http://openwrt.local/luci-static/video.html. The playback starts
as the browser now knows it is a video. However, if we now wanted to send
the video over Airplay to a second device, it's not gonna share the html
file found at url2, but rather the video file found at url1, and the
playback on second device will fail as it thinks it is a binary.

Adding Airplay supported extensions/mime types fixes the issue.

Signed-off-by: Sasha Andonov <s.andonnov@gmail.com>