Fixed extension when compute original chunks

[bsekachev]

Motivation and context

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #6355 (8080d2c) into develop (5d36068) will decrease coverage by 6.94%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6355      +/-   ##
===========================================
- Coverage    81.68%   74.74%   -6.94%     
===========================================
  Files          321      145     -176     
  Lines        37329    16934   -20395     
  Branches      6756     3285    -3471     
===========================================
- Hits         30493    12658   -17835     
+ Misses        6527     4270    -2257     
+ Partials       309        6     -303     

Components	Coverage Δ
cvat-ui	74.74% <ø> (-2.35%)	⬇️
cvat-server	∅ <ø> (∅)