fix(storage): issue with file uploading

[vorobeyme]

• add new method for uploading files (crowdin.go)
• add error handling for nil file (storage.go)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 68.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 98.20%. Comparing base (e0bfa3d) to head (7c47ed8).

Files	Patch %	Lines
crowdin/crowdin.go	61.91%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   98.46%   98.20%   -0.26%     
==========================================
  Files          66       66              
  Lines        2914     2934      +20     
==========================================
+ Hits         2869     2881      +12     
- Misses         25       29       +4     
- Partials       20       24       +4     

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[andrii-bodnar]

@vorobeyme thank you!