https://software.intel.com/en-us/articles/intel-texture-works-plugin
this one worked for me and many others


^ Click save as type drop down menu and find this^

This is what it looks like for me. Photoshop CC 2017
You may need to do a similar process to even open these files to begin with in that case refer to the first picture and do the same but select "Open As..."
I will gladly take suggestions on other programs and edit this post as needed. I've seen a few programs that can view this type of compression but so far things like plugins for gimp I haven't found when I first went looking to figure this out.
I also hope someone will be able to tell whether or not it is possible to get the files to be previewed as a thumbnail in file explorer because idk why they are just blank thumbnails assigned to whatever program you default open them with (in this case just photoshop squares that say ps)
I've seen many people ask for this so hopefully placing it in tutorials will help stop more threads
Update Edit:
Another simple option. Really helps for those without photoshop is paint.net https://www.getpaint.net/download.html
Combined with this plugin https://forums.getpaint.net/topic/11173 ... 018-02-22/ Github link
You might have to rename a file as dds2 to open within paint.net and then save as dds2 to use all the available options but it can read and write these formats so that is good to add into the available options. Just rename to dds before putting back into game.