Fix: Filetree icons

[orronai]

• Removed unnecessary file types in the svg and the css files
• Generalized the icon files

[codecov]

Codecov Report

Merging #270 (83fb4d0) into master (1472481) will decrease coverage by 0.05%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   79.03%   78.98%   -0.06%     
==========================================
  Files          55       55              
  Lines        2333     2346      +13     
==========================================
+ Hits         1844     1853       +9     
- Misses        489      493       +4     

Impacted Files	Coverage Δ
lms/utils/files.py	77.77% <50.00%> (-22.23%)	⬇️
lms/lmsweb/views.py	88.92% <87.50%> (-0.38%)	⬇️
lms/lmsdb/models.py	90.44% <100.00%> (+0.02%)	⬆️
lms/models/upload.py	91.11% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1472481...3aa4993. Read the comment docs.