Question: Why doesn't my template show a thumbnail of TIF files used as template assets?
TIFF files can be cause some confusion because on the surface they may seem like any other raster image file. But they can be saved with all sorts of options from an external application, which can allow for inclusion of layers, and the ability to save with a thumbnail.
If a TIFF file is saved with this option enabled, then the thumbnail should show up in your template as you would expect. But not all applications that have the ability to create TIFF files have that option, like Photoshop does to include a thumbnail.