.ase: Aseprite files. They use a proprietary format so you will need Aseprite to view these files.
.psd: Photoshop documents. You can open these with Photoshop, GIMP, Photopea, and most common image editors.
.xcf: GIMP files. You can use other programs with plugins but it's probably easier to just use GIMP since it's free.
.pdn: paint.NET files. These also use a proprietary format and you will need paint.NET to open these files.
Raw files are sorted by pack, and further by type (blocks are grouped together, items are grouped together, etc).
If the file has "recreation" in its name, the file is not original and has been remade. This may lead to the file being slightly inaccurate to the actual version in the pack.
If a texture gets replaced that has a raw file here, the raw file here will get replaced too. If you want access to the original raw file, you can use the built-in GitHub commit history to browse old versions of this repository and download the files you want.
If you want to add a file here, simply ask on our Discord Server.