Fix mimetypes for m4a/flac/ogg in fs_storage backend

[whorfin]

mimetype adjustments weren't actually persistent, fix that
Fix .m4a declaration
Add flac and ogg

[codecov]

Codecov Report

Merging #40 (12231fb) into master (25e6a85) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   47.27%   47.25%   -0.02%     
==========================================
  Files          50       50              
  Lines       10826    10828       +2     
==========================================
- Hits         5118     5117       -1     
- Misses       5708     5711       +3     

Flag	Coverage Δ
unittest	47.25% <66.66%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
coherence/upnp/core/service.py	66.44% <0.00%> (ø)
coherence/upnp/devices/media_server.py	33.33% <ø> (ø)
coherence/base.py	58.23% <40.00%> (-0.44%)	⬇️
coherence/upnp/core/device.py	73.41% <66.66%> (-0.06%)	⬇️
coherence/backends/fs_storage.py	43.43% <78.57%> (+0.17%)	⬆️
coherence/upnp/devices/control_point.py	38.33% <100.00%> (ø)

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