Ensure .js mimetype is set correctly

[philippjfr]

Fixes #4080

[codecov]

Codecov Report

Merging #4118 (f3c68f3) into master (1f3d889) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #4118   +/-   ##
=======================================
  Coverage   83.56%   83.56%           
=======================================
  Files         223      223           
  Lines       33256    33262    +6     
=======================================
+ Hits        27791    27797    +6     
  Misses       5465     5465           

Flag	Coverage Δ
ui-tests	37.56% <66.66%> (+0.02%)	⬆️
unitexamples-tests	74.65% <66.66%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/io/resources.py	86.39% <66.66%> (-0.42%)	⬇️
panel/io/state.py	69.81% <0.00%> (-0.19%)	⬇️
panel/tests/util.py	91.26% <0.00%> (+2.91%)	⬆️

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