fix(APIRequest): only use form data when actually sending files

[SpaceEEC]

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

This PR patches APIRequest to only make use of FormData when actually sending files.
This initially "broke" by APIMessage (consistently) returning an array for files even if no files are being sent.

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.