Add file type filtering in slash command attachment options and modal's fileupload component

[Soheab]

Summary

This PR adds supports for the functionality to limit file uploads from a modal or slash commands to certain extensions.

Docs PR: discord/discord-api-docs#8506

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• 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, ...)

[Rapptz]

Not big on the decorator design that only applies to a single parameter type. Unfortunately the alternative design space for this one would stick out as well.

[Soheab]

Not big on the decorator design that only applies to a single parameter type. Unfortunately the alternative design space for this one would stick out as well.

Yeah. An ext.commands like parameter() would've been better, but there is no such thing currently for slash commands.

[vmphase]

This is not documented in docs/api.rst