Ok I've updated it so it has an FBX file so if you redownload it now and do the same thing you did before it should show up no problem ready for upload :)
I figured out what it is, it's a blend file not an fbx file so just go into blender, open the blend file in there and go to - file - save as - and save it in the assets of unity document you want to use it in then when you go back on that unity document it should be there, let me know if this fixes your problem :)
Your Adventure Time avatars are freaking awesome, please would you consider making Flame Princess and/or Princess Bubblegum, a Marshall Lee would be awesome too, just some suggestions, I really like all your avatars :)