Add mgz to fileTypes

[jennydaman]

Possibly fixes a regression introduced in #1127

Some file preview components prefer to receive their data as a URI whereas some prefer to receive their data as a blob. In #1127 a hard-coded list of file extensions called fileTypes was added where files matching those extensions would always be loaded as a blob instead of by URI.

Loading data by URI is usually more efficient but it's not backwards-compatible with NiiVueDisplay.tsx, which wasn't updated in #1127. Here I did a lazy solution of adding mgz to fileTypes so that it'll work like it used to.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.83%. Comparing base (dac03ce) to head (5eac2b9).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1287   +/-   ##
=======================================
  Coverage   32.82%   32.83%           
=======================================
  Files         194      194           
  Lines        6400     6393    -7     
  Branches     1330     1327    -3     
=======================================
- Hits         2101     2099    -2     
+ Misses       4171     4166    -5     
  Partials      128      128           

Flag	Coverage Δ
e2etests-local	27.10% <100.00%> (+0.02%)	⬆️
e2etests-public	27.67% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.