Thumbnails

In Tiles, Icons and Gallery, file contents are shown instead of icons; Details keeps regular icons.

Files	Thumbnail
Images, HEIC, RAW, PSD	The image itself; RAW from the embedded preview
Video	A frame
FB2 and EPUB books, PDF	Cover or first page
Music	Cover art from the file or an image next to it (`Cover.jpg`, `folder.jpg`)
3D models	The model at a three-quarter angle; one you rotated in the Preview pane, at that angle; for 3MF, the slicer's picture of the plate
Fonts	“Abc” and “Абв” in the font itself
Office and OpenDocument files, Krita, OpenRaster, CBZ comics	The picture the program saved in the file; for CBZ, the first page
Folder	An overview of its contents
Shortcut	The thumbnail of what it points to

All formats are listed on the “Formats” page.

Speed: thumbnails are cached, so a folder draws faster when you come back to it. Cache size and memory for images are set in “Settings” → “Cache and memory”.
Freshness: thumbnails are regenerated when a file changes.