How to force rebuilding Document Previews

The easiest way is to go to Administration > File Viewers > Document Viewer > Documents, find the document, select it, and click "Reprocess." 

That would be good if we were talking about a handful of documents. What I am running into is with the upgrade from 8.5 to 10.1 the previews are all trashed and need to be regenerated. It is 10's of thousands of them. So I would like a way to get them all regenerated as quickly as possible. Particularly because the place where the thumbnail should be in the media gallery file list shows as a URL to the graphic that doesn't exist until the preview is regenerated. It just doesn't look good. 

That would be good if we were talking about a handful of documents. What I am running into is with the upgrade from 8.5 to 10.1 the previews are all trashed and need to be regenerated. It is 10's of thousands of them. So I would like a way to get them all regenerated as quickly as possible. Particularly because the place where the thumbnail should be in the media gallery file list shows as a URL to the graphic that doesn't exist until the preview is regenerated. It just doesn't look good. 

To fully regenerate everything, you can delete the docpreview and docpreview.s folders in your CFS location and truncate the telligent_DocumentPreview_Files table. The documents will be reprocessed as they are requested to be rendered in the UI.

Am I correct in assuming that the initiation of the preview creation process is on first access of the document? Does accessing the thumbnail of the preview in the file list page also trigger it? Or is is just when the file page itself is accessed? 

Accessing the preview or the full view will schedule the document for processing.