Add slash command attachment example

[krittick]

Summary

This adds an example for adding an attachment as a slash command option type.

This also fixes the missing SlashCommandOptionType reference in __all__ in enums.py

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, ...)

[VincentRPS]

@krittick why not just add this to the options example?

[krittick]

@krittick why not just add this to the options example?

Since you have to do some additional processing to make it work (e.g. to_file()), I felt that it's better as a standalone example.

[VincentRPS]

@krittick why not just add this to the options example?

Since you have to do some additional processing to make it work (e.g. to_file()), I felt that it's better as a standalone example.

it's still a small enough example where it would be better there

[krittick]

@krittick why not just add this to the options example?

Since you have to do some additional processing to make it work (e.g. to_file()), I felt that it's better as a standalone example.

it's still a small enough example where it would be better there

Sure, I've updated it now.