"Office XML Handler" is the display name of the process
MSOXMLED.EXE, which is locaded in
C:\Program Files (x86)\Common Files\Microsoft Shared\OFFICE15 on my computer. It is used to open XML files (perhaps also other extentions?) and detect which MS Office program the file should be associated with (if any), and then open the file with that program.
This is explained in https://stackoverflow.com/a/1569619/1858923:
So when an XML file is double-clicked in Explorer, Windows will launch MSOXMLED.EXE. This application is now looking inside the XML file and searches for an XML processing instruction. This processing instruction named mso-application can specify a ProgId:
So finally MSOXMLED.EXE will start the appropriate Office application or launch the default XML application which is specified under
Clearly I got the error message because the file it was trying to open did not have an
mso-application tag that would tell
MSOXMLED.EXE how to open the file, and I don't have the key
HKEY_CLASSES_ROOT\XEV.GenericApp\shell\open\command set, so it doesn't know what to do with the file.