Sorry for month late response but this is a known issue that began after the x64 bit update. The only work around is to compile the hat and then preview it in TFMV. Sorry :/

Ayy,

I've also been struggling with this for a while, you might have already find a solution but here is how I managed to do it, I don't think it's flawless though, I still have some issue and I can't tell if it's because the method I'm using is not the right one :

- Preview item doesn't seem to work
- TFMV also doesn't work and it looks like it would need some annoying modification to fix to make it work. But judging by all the post I have seen discussing about this, most of the solution seems to be only working for a few people, and are very hacky.
- I've tried through workshop.tf to upload my hat so I could preview it in loadout.tf, but the upload feature doesn't work at all for me.

What worked for me is this :
- In TF2 after you are done filling the information to compile your hat, do not click on Preview. Instead, click on Verify, and then Publish. That should generate multiple file. Most notably, it also generate a few or one MDL file in tf/models/workshop : MDL files are the format used to compile a hat into being readable for Source.

Remember where those MDL files are saved at, and then open TF2, open the console, type in "map itemtest", once you are in the map, type in console "itemtest", and then through that itemtest window you can spawn in a bot that is wearing your cosmetic by giving it the MDL file.

The only problem I have right now is that it seems that the game renders the surface that are facing inside (instead of outside), trying to fix it as I write this. good luck.