Allow read/write for .MID and .midi files

[VasanthManiVasi]

Hi, previously we were able to read .mid files and write both .mid and .MID files. Now we can read/write .mid, .MID and .midi files.
.MID and .midi are also quite common so it would be great to be able to read/write them. The extensions differ only by their name so the filename check was alone changed.

[Datseris]

Thanks for this. Perhaps it is about time to extend FileIO.jl for MIDI files. We could then export its save and load functions that dispatch on "magic numbers" for .mid endings. https://github.com/JuliaIO/FileIO.jl

Not really a priority, but if you are interested feel free to have a look at the FileIO docs and open a pr here and there, it is surprisingly simple.

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(also please increment patch number here)

[VasanthManiVasi]

Sure, I'll take a look at FileIO.jl. They allow passing a list of extensions when registering a new file format so we can add .mid, .MID and .midi too.