better file attachments

[krissetto]

• Inline text based files instead of using dedicated file api for now (can be improved to detect text files and send the standard plaintext content type instead, but this is at least standard and will work the same for all providers right now)
• Don't strip placeholders in msg text, they can be useul when referencing multiple files
• Fix double @@ when using the file picker for an @ attachment

Fixes these issues seen in the screenshots below:

• text files not ending in .txt were being skipped;
• .md files were being rejected by anthropic's file api
• placeholders for the attachments were no longer showing in the TUI after sending the message

[github-actions]

Code Review Summary

I've reviewed the changes for better file attachment handling. The implementation looks solid overall - the shift to inlining text files directly instead of using provider-specific file APIs is a good approach for cross-provider compatibility.

Key Changes Verified

✅ Text files are now inlined with XML-like tags for context
✅ Binary files (images, PDFs) remain as file references
✅ Comprehensive test coverage added
✅ Proper error handling for file operations
✅ Size limits enforced (5MB for inline text files)

Minor Note

There's a theoretical TOCTOU (Time-of-Check-Time-of-Use) race condition where a file could grow between the size check and the actual read operation. However, this has minimal practical impact because:

• The application is a local TUI/CLI tool operating on user-provided files
• Error handling properly catches file access failures
• The exploit window is microseconds and requires filesystem write access
• Size checks prevent most DOS scenarios

The code is well-structured with good error handling and user warnings. Nice work!