Accessiblity: Img tags missing alt tags + tests

[timotheeguerin]

fix #1482

[codecov]

Codecov Report

Merging #1483 into master will increase coverage by <.01%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master    #1483      +/-   ##
==========================================
+ Coverage   59.05%   59.06%   +<.01%     
==========================================
  Files        1053     1053              
  Lines       24696    24708      +12     
  Branches     3414     3414              
==========================================
+ Hits        14585    14593       +8     
- Misses      10111    10115       +4

Impacted Files	Coverage Δ
...s/image-file-viewer/image-file-viewer.component.ts	100% <100%> (+4.34%)	⬆️
app/services/file/file-loader.ts	12.5% <11.76%> (+0.37%)	⬆️

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