Add files upload examples and fix form data header

[asimonok]

Resolves #252
Resolves #316
Resolves #353
Resolves #249

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (d26cdca) to head (2943d0e).
Report is 1 commits behind head on main.

❗ Current head 2943d0e differs from pull request most recent head 296559a. Consider uploading reports for the commit 296559a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
+ Coverage   97.98%   98.03%   +0.04%     
==========================================
  Files          66       68       +2     
  Lines        1144     1168      +24     
  Branches      210      214       +4     
==========================================
+ Hits         1121     1145      +24     
  Misses         13       13              
  Partials       10       10              

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[mikhail-vl]

Looks great!