Hey! Thanks for reporting this—I think there are a couple things going on here. Mailspring doesn’t preview TIFF images right now, and also doesn’t preview images that are larger than 5MB (because they can make the app use a lot of memory.)
I think it’d be fairly safe to allow <5MB tiff files to be previewed along with JPEG, GIF, etc. If anyone wants to take a look, I think this might be a good starter issue! There’s a utility function called shouldDisplayAsImage that determines whether Mailspring treats an attachment as an image or a file, I think the change could go in there.
(Originally posted by bengotow on GitHub.)