OpenAI GPT models reject Desktop image attachments sent as application/octet-stream #37581

Description

When I attach an image while using any model in the OpenAI GPT family in OpenCode Desktop, the request fails with:

file part media type application/octet-stream functionality not supported

This occurs across the entire GPT model family, not just one specific model. The image appears to be sent as application/octet-stream instead of its actual MIME type, such as image/png.

The same image can be accessed when I provide its full local path and ask the agent to read it. Only direct attachment through OpenCode Desktop fails.

Expected behavior:

The image should be sent with the correct MIME type and passed to the selected GPT model.

Actual behavior:

The image is sent as application/octet-stream and the request is rejected.

Plugins

@prevalentware/opencode-goal-plugin

OpenCode version

1.18.3

Steps to reproduce

1. Open OpenCode Desktop on Windows.
2. Select any OpenAI GPT model that supports image input.
3. Attach a PNG image using the attachment button.
4. Ask the model to describe the image.
5. Send the message.
6. The request fails with: "file part media type application/octet-stream functionality not supported".
7. Repeat with other GPT models and observe the same error.

Screenshot and/or share link

No response

Operating System

Windows 11

Terminal

OpenCode Desktop