fix(cli): allow document uploads in buzz upload file

[ajaym94]

buzz upload file and messages send --file only accept jpeg/png/gif/webp/mp4, so agents can't share a PDF, a Markdown file or a plain text file, even though the relay stores generic attachments on /upload and Desktop already allows them.

This makes the CLI follow the relay's rules: media keeps its current caps, other image/video/audio types and the relay's blocked types are still refused locally, and everything else goes up with a 100 MiB cap. Generic files now carry filename in imeta, and messages send --file links them as [name](url) instead of ![image](url), so they show as a file card with the real name instead of <hash>.bin.

Tested with cargo test -p buzz-cli and by uploading a .md and a .pdf to a self-hosted relay.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 2664d14316790a57ea44b3f97c57440b70e43436...6aa5d4666f9bead4d24d9c3de2c95e738f5e02c1.
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