Attached AVIF images are silently dropped from model context in V2

Summary

In OpenCode V2, pasting or @-mentioning an AVIF image attachment in the TUI inserts an [Image N] mention label, but the image is silently dropped when building the model request: to-llm-message.ts only forwards image/png, image/jpeg, image/gif, and image/webp as media, and attachmentContent returns [] for any other image mime. The model sees the [Image N] label but never receives the image content, so it may hallucinate or fail to work with the attachment. The TUI file picker explicitly supports .avif (see local-attachment.ts mime table), so the format is accepted on input but vanishes on output.

Environment

• opencode version: v2 (next)
• OS: macOS
• Install/channel: next

Reproduction

1. In the V2 TUI, paste a local .avif image path (supported by the attachment picker)
2. The TUI inserts [Image 1] and shows the attachment
3. Submit the prompt and inspect the model request: no media part and no text fallback for the image

Related: #17772 requested AVIF support or auto-conversion; the format is still dropped today. Unsupported formats like image/tiff hit the same path.