Add support for files with .aac extension

I raised this issue some time ago on your old bug tracker (https://bugs.kde.org/show_bug.cgi?id=298198) and finally take some time to try to fix it myself.

More details are available in the original bug but, to summarize:
- support for file with .aac extension by adding a special check, like for .oga, as .aac files might corresponds to MP4 or MPEG (http://en.wikipedia.org/wiki/Advanced_Audio_Coding#Container_formats).