A preview is the browsing thumbnail for a video or 3D model. It is separate from the original file. A missing preview does not by itself mean that the original failed to upload.
When previews appear
Video preview work starts after the MP4 is validated. A 3D file may initially show a placeholder. Opening a video or model's details with edit access requests a preview if a current one is missing.
You can attach and open a validated original while its preview is being prepared. The library may show a video or 3D placeholder until the thumbnail is ready.
If a preview is unavailable
Open the file details and check the original. If Retry preview is available, use it to request the thumbnail again; edit access to that library is required. A pending preview can finish after you close the details.
A valid file may still contain content the preview renderer cannot display. If a model repeatedly has no preview, check that it contains renderable geometry and that its required resources are included in the GLB or USDZ file.
Requesting a preview does not upload another original or replace the file already in use. Deleting the file also removes its preview.