use video/webm instead of audio/webm for webm

use video/webm instead of audio/webm for webm

StreamFile::contentTypeFromPath via MimeMagic::guessTypesForExtension
returns the first mime type for a given extension listed in
includes/mime.types. This is used to set the mime type of files in
SWIFT. While audio/webm is a valid mime type for .webm files,
we only have videos with .webm right now. moving video/webm
up to make it the default mime type.

This fixes issues with players that will only show a video if
type is video/webm: Bug 43476

Change-Id: Ib4868995a9b2f474e656ecc4b23a18201fbeb572