feat: ability to properly upload file attachments

[MiniatureEge2006]

Problem

I want the ability to actually upload files to the composer without having to paste or drag and drop it.

Proposed solution

Either a concord native slash command that allows you to pass single or multiple file paths or an interactive picker.

Alternatives considered

No response

Additional context

No response

Confirmations

• I searched existing issues and discussions for the same idea.
• This request does not involve self-bot automation, mass actions, or scraping.