This will simplify testing :^)

Priority is per-(direct subclass of)FileLoaderBase -- i.e., each
concrete subclass of, say, SoundFileLoader must have a unique priority.

See #124 for more context :^)

Turns out that the builtin `mimetypes` module can't detect using
anything except file extension, so that's basically useless for us.

This also adds tests from the ftyers/ directory.

Before, we were relying on garbage collection to clean up our temp
files, which is probably not tenable long-term.

There needs to be some class sitting between SoundFileLoader and the
concrete children since
 - each child shares a lot of code with siblings
 - each child needs the `pydub` external library, which may not exist
   on all systems

See #122 for more context.