## Summary
Fixes#16900 - MS Office documents (doc, docx, ppt, pptx, xls, xlsx,
odt) cannot be previewed when hosted on local/private networks.
## Problem
Microsoft's Office Online viewer (`view.officeapps.live.com`) requires
documents to be publicly accessible from the internet. For self-hosted
Twenty instances or local development, files are not reachable by
Microsoft's servers, causing the viewer to fail with "An error
occurred".
## Solution
Detect private/local URLs upfront and show a helpful message with a
download button instead of letting the viewer fail silently.
**Detected private URLs:**
- `localhost`, `127.0.0.1`, `[::1]`
- Private IP ranges: `10.x.x.x`, `172.16-31.x.x`, `192.168.x.x`
- Local domain patterns: `.local`, `.localhost`, `.internal`
## Alternatives Explored (that don't work)
We investigated more sophisticated detection approaches:
1. **postMessage events** - Microsoft's viewer doesn't send any error
messages we can listen to
2. **Fetching the embed URL** - Blocked by CORS (Microsoft doesn't set
`Access-Control-Allow-Origin`)
3. **Reading iframe content** - Cross-origin restrictions prevent
JavaScript access
The URL-based detection is the most reliable approach for catching the
common cases where preview will definitely fail.
## Testing
1. Upload an Office file (docx, pptx, xlsx) on a local Twenty instance
2. Try to preview it
3. Should see "This file cannot be previewed because it is hosted
locally" with a Download button
