Pasting an image file loses the original path — model only gets an inline data: URL

Description

When an image file is pasted into the TUI composer (pasted path text, or a Finder file copy), the attachment is always submitted as an inline data: URL. The original path only survives in the part's source.path metadata, which the model never sees. After materialization the model effectively has an image and a temp path, so it cannot reference the source file it was given.

The server already does the right thing for file: URL attachments: in packages/opencode/src/session/prompt.ts it emits a synthetic text part with {"filePath": ...} alongside the image. The TUI paste flow just never produces file: URLs, so that path is unreachable.

Secondary symptom on macOS: pasting with the internal paste command (prompt.paste) with a Finder file copy on the clipboard attaches the file's generic icon bitmap, because clipboard.read() requests the clipboard as "PNGf" and the icon flavors win over the file URL/text flavors.

Steps to reproduce

1. Copy an image file in Finder
2. Paste it into the TUI composer (it is auto-attached as [Image 1])
3. Submit and ask the model for the path of the attached image
4. The model has no path: the part is a data: URL, materialized to a temp file

OpenCode version

v0.0.0-beta-19381 (also present on dev)

Operating System

macOS