bug-909373-[Music] open activity saving feature robustness r=dkuo

in some cases for example `ogg` files, `guessTypeFromExtension(ext)` could return video instead of audio. To overcome this problem, but still check file extension is valid, I get the file type from extension and then check that it is audio or video. I am assuming any file that music player will be able to open must be one of these two.

* Note: this does not handle the edge case where open activity is called with a music file, but provides another music file type. For example `mp3` file claiming to be `ogg`.

I submit this PR so we can discuss if this is sufficient check or if we need to add more validation to cover the above case