As suggest above, you may try running changing the output to excel when submitting the request, also in addition please also see:
Unable To Get Excel Output From A RDF File And XML Publisher, HTML Code Shows Up As Output (Doc ID 1231163.1)
How to Create R12 XML Report Output Formatted in Excel Instead of PDF (Doc ID 568031.1)
Changing Output Format to RTF or Excel in XML Publisher Request Does Not Display Output Correctly (Doc ID 404512.1)
Octopus Rex - This program is different, It has a parameter for specifying the XML template. So, in the Upon Completion section, Layout is empty and we cannot specify any output format either.
@AsifMuhammad - Thanks for the tips, but none of these is the issue.