Tells me:
Unknown file type: .
It must be either a head, body, shield or sword.

What do I do? I don't understand as I am uploading a head template?  

I downloaded the head file as a .JPEG and then opened it in GIMP 2 and re-saved it as a .PNG and it allowed me to upload it and looks like it worked for you too, so if anyone has this issue I guess try doing this as well.

if you don't have GIMP2 it will probably work in any other graphics program as well (Photoshop, PAINT.net, w/e other ones you guys use)