Disable thumbnail upload

My aim is to use this plugin to upload only PDF images to an S3 bucket.

I am using the as3cf_allowed_mime_types filter to suppress the upload of image file types including jpg,jpeg and jpe types to the bucket.

This seems to be working, however jpg thumbnails generated for each PDF uploaded to WordPress seem to be being uploaded to the bucket too, and I would rather they stayed on the server.

Any suggestions on how to stop the thumbnail upload gratefully received.