Fix #728 by adding bytearray support in files_upload (sync mode)

[seratch]

Summary

This pull request fixes #728 by adding bytearray type support for file-type parameters. This issue has been an incompatibility issue when using run_async=False (the default) mode since v2.6.0

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #730 into master will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
- Coverage   85.21%   85.15%   -0.07%     
==========================================
  Files          22       22              
  Lines        2800     2802       +2     
  Branches      280      281       +1     
==========================================
  Hits         2386     2386              
- Misses        310      312       +2     
  Partials      104      104              

Impacted Files	Coverage Δ
slack/web/base_client.py	71.56% <0.00%> (-0.47%)	⬇️

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