SharePoint Online problem with File Viewer Thumbnail

In my SharePoint Online site, I have multiple pages with the File Viewer web part. When the page is loading, a thumbnail of the file is being displayed for a few seconds.

If the file is a video, the thumbnail that SharePoint displays is a random screenshot of it, like below. I would like to have the ability to change that to display a more corresponding image.

If the file is a document, like a PDF or a word document, I have a blurry image as a thumbnail which is displayed for a few seconds until the page is fully loaded. I would like to be able to change the size or the thumbnail itself because this is confusing and looks like an error for the user.

I have tried to change the thumbnail by creating a “Thumbnail” column in the document library and changing the path there, but SharePoint seems to ignore the set value.  I have also tried to change it with PnP with the command

Set-PnPListItem -List xxxx -Identity xxxxx -Values @{"Thumbnail"=""}

 but doesn’t seem to work either.

Has someone dealt with this before? Do you know of a way to handle thumbnails to File Viewer in SharePoint Online?