This upscaler is not mine, all the credit goes to: KIM2091
Official WIKI page: openmodeldb
License of use it: CC-BY-NC-SA-4.0
HOW TO INSTALL:
Rename the file from: ultramix_Smooth.pt
to 4x-UltraMix_Smooth.pth
Copy the file 4x-UltraMix_Smooth.pth
inside the folder: stable-diffusion/models/ESRGAN
Restart/Reload you Stable Diffusion instance
The upscaler should be visible for hi-res and extras tabs