Implemented attachments in Webhook.edit_message and WebhookMessage.edit

[Vioshim]

Summary

This allows webhooks to be able to edit attachments according to the implementation in Discord's documentation

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

[plun1331]

Passing files to this without attachments would replace all of the message's attachments with the files passed.
Other than that, looks fine.

[Vioshim]

Passing files to this without attachments would replace all of the message's attachments with the files passed. Other than that, looks fine.

As shown in the Discord server discussion, given that the payload never adds the parameter attachment in such cases, it shouldn't have such issue given how the API works, instead it'd simply append files in the message. See Documentation as Reference

The replacement would only happen if the user provides a list of attachments, even if it's zero length.

[Lulalaby]

Passing files to this without attachments would replace all of the message's attachments with the files passed.

This is the future plan of discord.

The replacement would only happen if the user provides a list of attachments, even if it's zero length.

For now correct.
It changes tho in v10

[krittick]

Since this is existing API functionality that we currently lack, I'm good with merging this now and then updating for API v10 once that arrives.