Xtractr should use file content to determine archive type

Hello,

I had an issue with unpackerr due to one of the *Arrs where an archive was mislabelled as RAR, but it was in fact a 7z archive. This is easily identified using the file command: file actually_7z.rar. Archives generally have magic numbers to identify their type more reliably, I think this is a less brittle method.