refactor: rename attachment to file

[ckohen]

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

Discord and djs both use files to classify all attachments, we should follow suit.

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 #98 (083148d) into main (9070440) will not change coverage.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##             main      #98   +/-   ##
=======================================
  Coverage   88.82%   88.82%           
=======================================
  Files           9        9           
  Lines        1584     1584           
  Branches      167      167           
=======================================
  Hits         1407     1407           
  Misses         13       13           
  Partials      164      164           

Impacted Files	Coverage Δ
packages/rest/src/lib/RequestManager.ts	93.09% <80.00%> (ø)
packages/rest/src/lib/REST.ts	95.57% <100.00%> (ø)
packages/rest/src/lib/errors/DiscordAPIError.ts	82.24% <100.00%> (ø)
packages/rest/src/lib/errors/HTTPError.ts	96.66% <100.00%> (ø)
...ackages/rest/src/lib/handlers/SequentialHandler.ts	82.78% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9070440...083148d. Read the comment docs.