Fix: gracefully error out for unsupported images

[jobchong]

Fix for #7459

What

Since codex errors out for unsupported images, stop attempting to base64/attach them and instead emit a clear placeholder when the file isn’t a supported image MIME.

Why

Local uploads for unsupported formats (e.g., SVG/GIF/etc.) were dead-ending after decode failures because of the 400 retry loop. Users now get an explicit “cannot attach … unsupported image format …” response.

How

Replace the fallback read/encode path with MIME detection that bails out for non-image or unsupported image types, returning a consistent placeholder. Unreadable and invalid images still produce their existing error placeholders.

[jif-oai]

@codex review

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. Keep it up!

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[etraut-openai]

Thanks for the contribution!