Improve management of instrumental files

[Neraste]

• Detect files by extension first;
• Detect MKV audio files (.mka) as audio file. Note: it seems that the MIME type depends on the OS, and the .mka file may not be listed in the types. To circumvent this, we’ve introduced hard-coded, "known extension" types, for usual files we know;
• Transmit instrumental file and instrumental track fields to server.

Related to:

• Improve management of instrumental files dakara-server#190
• Improve management of instrumental files dakara-player#143

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ef7f616) to head (28e2807).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        20    +1     
  Lines          831       859   +28     
=========================================
+ Hits           831       859   +28     

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