chore(RequestManager): use new file upload format

[ckohen]

Please describe the changes this PR makes and why it should be merged:

Ported from discordjs/discord.js#6871

Co-authored-by: Jake Ward geek@gaminggeek.dev

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[codecov]

Codecov Report

Merging #88 (3b9eca2) into main (c22081d) will decrease coverage by 0.11%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   89.03%   88.91%   -0.12%     
==========================================
  Files           9        9              
  Lines        1487     1489       +2     
  Branches      153      155       +2     
==========================================
  Hits         1324     1324              
  Misses         12       12              
- Partials      151      153       +2     

Impacted Files	Coverage Δ
packages/rest/src/lib/RequestManager.ts	93.42% <60.00%> (-0.54%)	⬇️

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