By configuring the thumbnails, do you mean the ability to configure the styling of the gallery item thumbnails in the 'Map focused' app layout? If so, there have been new setting enhancements added to the Attachment Viewer's configuration panel which provides the option to easily style the thumbnail's format and height, labelled as 'Thumbnail format' and 'Thumbnail height', respectively.
These settings can be found in the 'Theme & Layout' section.
The thumbnail items can also be re-styled using the 'Custom CSS' option.
Please let me know if you have any additional questions!
Even though I can change the thumbnail size, I can't change the thumbnail. All my attachments are pdfs and thus appear like this:
Is there any way to set the thumbnail image?