Update filemagic to new API to fix meme crash

[clintgibler]

When running scripts/promnesia index on macOS, I received the following error:

TypeError: __init__() got an unexpected keyword argument 'mime'

From src/promnesia/sources/auto.py#L22:

@lru_cache(1)
def _magic():
    import magic # type: ignore
    return magic.Magic(mime=True)

Reviewing the filemagic docs, as linked by @flinge in #122, I was able to fix the error by changing the arguments to the magic.Magic constructor and use magic.id_filename() instead of magic.from_file().

Notes

I'm not familiar with the Promnesia code base nor filemagic, so honestly I'm not sure if this code behaves as it should. Would greatly appreciate your feedback 🙏

Also, from the filemagic docs,

To ensure that resources are correctly released by magic.Magic, it’s necessary to either explicitly call close() on instances, or use with statement.

Since _magic() returns an instance of magic.Magic without a close() or using with, the code may currently be leaking file descriptors, though I'm not sure.

It would probably be good to use a with statement unless there's some huge performance or other impact I'm not aware of. This PR does not use with because I was trying to make the diff as small as possible.

[karlicoss]

Thanks! Appreciate the fix, but let me know if using python-magic solves this for you #122 (comment)

But well spotted about closing -- it seems that python-magic doesn't implement a context manager, even though in principle it probably should (it relies on __del__ at the moment https://github.com/ahupp/python-magic/blob/master/magic.py#L118-L131). I guess this is not super critical though -- even if it's leaking some resources, they will be released when indexing is finished along with the whole process. Perhaps this will matter more later if realtime indexing is added.

[clintgibler]

It does! I commented as well: #122 (comment). Closing this PR.

I see you beat me to adding a link to this on #20 :)