I love the beta thumbnailer, but I think this feature will make it even better by differentiating .ASE files from other image files that would open in a normal image viewer.
Basically, windows has this tickbox “Display file icon on thumbnail” in its folder options that shows small previews of the file type in the lower right corner of the thumbnail, so you know what extension the file has without reading its name (especially if you have extensions disabled). I searched everywhere for a way to turn it on for .ASE files, but it seems it can only be turned on by the thumbnailing shell extension (the aseprite thumbnailer, in this case, correct me if I’m wrong).
For examples, look at VLC’s thumbnails (showing the little cone in the corner), .KRA files from Krita and the .SVG thumbnailer from Inkscape.
P.S. For those who didn’t know Aseprite had a thumbnailer: Aseprite - Thumbnails