select transcoding not based on file extension

[piethein]

I have m4a files in my digitized audio collection. some of them are encoded with AAC, some with Apple Lossless. I wonder if it is possible to transcode only the files encoded in Apple Lossless, and not transcode the AAC files. I could of course change file extensions to reflect different codecs, but I believe it would be neater to have subsonic(/ffmpeg?) recognize the contents of the m4a file and decide what to do accordingly. Another possibility that would work fairly well would be to transcode depending on the bitrate of the source file (for example don't transcode below 384kbps)