[FEATURE]: Desktop should accept more dropped file types in prompt input #18143

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

This is a Desktop-specific follow-up to the broader file drag-and-drop / attachment issues already reported in this repo.

In OpenCode Desktop, dragging certain file types into the prompt input shows this error instead of inserting the file as a prompt reference:

Unsupported attachment
Only images, PDFs, or text files can be attached here.

That should not be the behavior for this workflow, and it is not the behavior in TUI. The wider problem is that Desktop currently accepts fewer dropped file types than TUI, so files that should be usable as prompt context are rejected before they can be inserted.

Expected behavior:

• Dragging supported local files into Desktop should insert them into the prompt input as references/context instead of rejecting them.
• Desktop should support the same or similar prompt file drop behavior as TUI where possible.
• The Desktop prompt should not block common developer file formats just because they are not images, PDFs, or plain text.

Actual behavior:

• Desktop rejects various dropped file formats with the unsupported attachment error.
• The file is not inserted into the prompt input as a reference.

Why this issue is different:

• This is specifically about the Desktop app's attachment restrictions and mismatch with TUI behavior.
• Existing issues touch adjacent areas, but this issue is meant to track the broader Desktop gap where dropped files are not accepted into prompt input as references.

Related issues:

• [FEATURE]: Edit Prompts and File Drag-and-Drop. #15493 - Desktop app only allows files from the active working directory / Desktop-specific drag-and-drop gap
• [FEATURE]:It's recommended to drag and drop files in, and it should support more formats, such as .kt, .java, .xml, etc. #15177 - More drag-and-drop file formats requested
• [FEATURE]: improve TUI clipboard paste and drag-and-drop file handling on Linux #14555 - TUI/Linux drag-and-drop and paste handling improvements
• Images can not be dragged and dropped #4668 - Images can not be dragged and dropped
• Drag-dropped images lose absolute file path, preventing tools from accessing them #17488 - Drag-dropped images lose absolute file path
• [Bug] Image drag-and-drop not working in Cursor terminal — works in iTerm #18107 - Cursor terminal drag-and-drop image path fallback discussion

Additional context:

• Screenshot attached in the original report shows the Desktop error state.
• This request is mainly about parity with TUI for inserting dropped files into the prompt as references/context.