I am wondering if your Windows file associations are broken. File associations are stored in the wonky registry table Windows uses to connect file extensions to applications. Here's what I see when I open an command prompt (ArcGIS 10.7) and check to see what pitemx is associated with:
C:\Users\cprice>assoc .pitemx
.pitemx=ArcGISPItemXFile
If the associations are broken, the best fix is to do a repair install of ArcGIS, as the installer will reset file associations to what they should be for your application. Of course the system breaks down when two apps want to use the same file extension, but that's Windows!