Make Codex Desktop file preview size limit configurable

What feature would you like to see?

Codex Desktop currently blocks workspace file previews above a fixed 10.0 MB limit. Users should have a way to choose whether they want to preview larger local files, either through a configurable setting or a per-file "preview anyway" action.

Current behavior

When selecting a larger PDF in the Codex Desktop file tree, the preview pane shows:

File is too large to preview
46.7 MB exceeds the 10.0 MB preview limit

Example case: a local research-paper PDF named 2025_Brain_Mapping_with_Dense_Features_BrainSAIL.pdf is 46.7 MB, so it cannot be previewed inside Codex even though it can be opened externally.

Expected behavior

Please allow the user to opt in to larger previews. Possible approaches:

• Add a config setting, for example desktop.preview_max_bytes = 41943040 or similar, so users can raise the limit to 40 MB or another value.
• Add a Codex Desktop setting for "Maximum file preview size".
• Keep the 10 MB default, but show a "Preview anyway" button with a performance warning.
• Support type-specific limits, for example allowing a higher limit for PDFs than for raw text or other file types.

Why this matters

Many normal PDFs, especially research papers with dense figures, high-resolution scans, or image-heavy appendices, exceed 10 MB. For local trusted workspaces, the user is already choosing to inspect files on their own machine, so they should be able to trade off performance/memory risk against convenience.

This is especially useful for workflows where Codex is organizing, summarizing, or reviewing paper collections and the user wants to inspect the paper in the same app without jumping to an external viewer.

Reproduction steps

1. Open Codex Desktop on a local workspace.
2. Select a PDF larger than 10 MB from the file tree.
3. Observe that the preview pane refuses to render the file and shows the 10.0 MB preview limit message.

Environment observed

• Surface: Codex Desktop workspace file preview pane / side panel
• Platform: macOS
• Desktop bundled CLI observed locally: codex-cli 0.137.0-alpha.4
• File type: PDF
• Example file size: 46.7 MB

Related issues checked

I searched for existing issues using the exact phrases File is too large to preview, 10.0 MB preview limit, and too large to preview, and did not find a direct duplicate in openai/codex.

Adjacent PDF preview issues exist, but they appear to cover different problems:

• #26292: stale PDF preview after overwrite on Windows
• #23233: PDF font rendering issue
• #6409: original PDF preview/download feature request