The ArcCatalog Options has the ability to add file types that will display in the Contents. For example you could add PDF and see them listed in ArcCatalog. Clicking on it would open the file in the associated program.
This would be handy to have in Pro.
This came about because I noticed that aprx's weren't showing up in Catalog (I found this idea for that) and thought I could add it as a file type.