If your application handles files which could be printed or could be represented well as a document, adding a thumbnailer makes the files show up in Files with images that correspond to the document.
The gnome-desktop library defines the interface and associated files that a thumbnailer must implement. The component responsible for calling the thumbnailer is GnomeDesktopThumbnailFactory.
A common helper for thumbnailers, which handles the command-line parsing and output file generation, is gnome-thumbnailer-skeleton.
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