CLI: --file flag attaches images with incorrect MIME type, breaking multimodal support #24698

Description

When using the opencode run --file flag with an image file (such as PNG or JPG), the CLI sets the MIME type to text/plain regardless of the file extension. As a result, image attachments are not recognized as images by LLMs or the OpenCode server—they are not handled as multimodal inputs, so image models are not triggered as expected.

Attempts to work around this with a plugin are not effective, because the CLI constructs the "file" part with the wrong MIME type before any plugin hooks run. A proper solution would detect and set common image MIME types (e.g., image/png, image/jpeg) for recognized file extensions whenever the --file flag is used.

This limitation breaks workflows that need to send screenshots or reference images through CLI-driven code automation.

Plugins

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OpenCode version

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Steps to reproduce

1. Run opencode run --file path/to/screenshot.png "Describe this screenshot"
2. Notice that the image is sent as a plain file (MIME type: text/plain), not as an image/multimodal input.
3. No way to force correct MIME with only CLI options; issue persists even with plugins.

Screenshot and/or share link

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Operating System

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Terminal

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