If you take a look at the files attached to the description of https://avan.tech/item42073 for example, you will see that some of them have a "cogwheel in a file" icon instead of a thumbnail.
Also, in my case, clicking their filenames to download them did not directly download them (in Firefox) like the other images (that are regular JPEGs), the browser prompted me about what I wanted to do with them... but I'm not sure if it was Tiki doing something weird, or if it was just a wrong mimetype setting in my browser (setting it to default to "Save file" instead of "Show in Firefox" made it work)