Since from_buffer/from_file memoize their result, this is going to produce inconsistent results if you switch the file mid-execution. I'd prefer that this get solved either by globally overriding the file (there's an env var IIRC though I don't remember it off the top of my head), or by using the Magic class directly.

I was using the Magic class directly before, but it gives problems when you use it from different threads, which is something the module functions do in fact handle correctly.
Using the environment doesn't really sound like a very good option to me - when working from within Python.

Let me know if you care to reconsider this PR, otherwise I guess it's also fine ;-)