TODO comment regarding accessibility description fallbacks

Signed-off-by: Matthias Stevens <matthias.stevens@gmail.com>
Moved isAudioFileName() and isPhotoFileName() (renamed as isImageFileName()) out of FileHelpers and into new MediaHelpers class,
also changed it such that the regex patterns are no longer recompiled every time...

Signed-off-by: Matthias Stevens <matthias.stevens@gmail.com>