Simplify and make file extension handling more robust

[hackdna]

No description provided.

[codecov]

Codecov Report

Merging #3155 into develop will decrease coverage by 0.36%.
The diff coverage is 93.33%.

@@             Coverage Diff             @@
##           develop    #3155      +/-   ##
===========================================
- Coverage    61.64%   61.27%   -0.37%     
===========================================
  Files          440      440              
  Lines        27981    27695     -286     
  Branches      1281     1280       -1     
===========================================
- Hits         17248    16970     -278     
+ Misses       10733    10725       -8

Impacted Files	Coverage Δ
refinery/file_store/test_models.py	100% <100%> (ø)	⬆️
refinery/file_store/models.py	79.48% <81.81%> (-5.56%)	⬇️
refinery/file_store/utils.py	51.21% <0%> (-13.79%)	⬇️
refinery/file_store/tasks.py	51.13% <0%> (-12.67%)	⬇️
...ce/js/dashboard/controllers/data-sets-card-ctrl.js	69.02% <0%> (-1.06%)	⬇️
...urce/js/dashboard/controllers/history-card-ctrl.js	92% <0%> (-0.31%)	⬇️
...ce/js/dashboard/controllers/dashboard-main-ctrl.js	95.83% <0%> (-0.17%)	⬇️
refinery/core/views.py	48.5% <0%> (ø)	⬆️

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