Looking at your .zip, you did have a file with a .ico extension that was NOT in .png format. I opened it in GIMP and saved it as .png after deleting the irrelevant layers. Finally, as Aranuvir told you, I changed the .png extension to to .thumb.
I think the only .png in your .zip was the material texture, not the preview icon. I am attaching the converted .ico file for you to rename yourself because ironically the forum doesn't seem to accept the .thumb file extensions. At least now have fixed your OS so you can see extensions.
- thumbnail
- boxer_briefs_ii.png (8.21 KiB) Viewed 7334 times