improve contentType detection

[jackrobison]

fixes #1690

[codecov]

Codecov Report

Merging #1700 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1700      +/-   ##
=========================================
+ Coverage   71.29%   71.3%   +<.01%     
=========================================
  Files         156     157       +1     
  Lines       14112   14116       +4     
=========================================
+ Hits        10061   10065       +4     
  Misses       4051    4051

Impacted Files	Coverage Δ
lbrynet/extras/daemon/mime_types.py	100% <100%> (ø)
lbrynet/extras/daemon/Daemon.py	50.25% <100%> (ø)	⬆️
lbrynet/extras/daemon/Publisher.py	85.1% <100%> (-0.61%)	⬇️

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