Hi,
Yep, once the Corona library materials are updated to Cosmos, the textures will indeed be in (.tx format) which is a bit different from the usual image files you might be used to. But don't worry, you can still open and edit these in Photoshop. Just use the "Open as" feature in Photoshop and select TIFF as the file type. This way, you can work on the .tx files directly in Photoshop.
Keep in mind, though, that if you're planning to make changes to the textures found in the Cosmos folder, after editing, when you save them back, you will lose mipmapping of the texture.
If you're looking to maintain the mipmapping, then try to do the following.
open as tiff -> edit -> save as some other losless compression format -> convert to tx using maketx tool which can be found here(
https://docs.chaos.com/display/VMAX/Misc+Tools#heading-maketx1412)
I hope this helps.
