BSP agent -> Pinner multipart upload fix

[tarassh]

WriteField should be used only for text fields.
CreateFormFile + part.Write send binary content.

PS. Pinner has also to be adjusted.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 15.52%. Comparing base (d0f896c) to head (7908393).
Report is 9 commits behind head on develop.

Files	Patch %	Lines
internal/storage/ipfs.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #280      +/-   ##
===========================================
- Coverage    15.54%   15.52%   -0.02%     
===========================================
  Files           53       53              
  Lines         6171     6176       +5     
===========================================
  Hits           959      959              
- Misses        5164     5169       +5     
  Partials        48       48              

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[noslav]

@tarassh Closing in favor of merged #282. All new PRs should always point to develop (and branch off from there as well) from where we can test it in the ovh test env