@jneilliii ,
First, I very like the Slicer thumbnails plugin.
For some reason I had to upload a gcode file through SFTP and not with the Octoprint UI.
As I understand the Slicer Thumbnail plugin generates a PNG file in ~/.octoprint/data/prusaslicerthumbnails and write a link in .metadata.json after the upload with the Octoprint UI.
I could do it manually.
But do you know a way to trigger the thumbnail generation if the file was uploaded with another way?
There's an option in the plugin's settings Rescan all files when pressing refresh in file list., or from within the settings you can trigger a crawl of the files to extract.
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I went to the settings before but read it really too quickly. 
Sorry to have bothered you.