Resolution: Not A Defect
If the file type is considered as binary by Jenkins webserver then the TAP plugin doesn't consider this file for inclusion in TAP reports. This can be quite confusing and if you're not aware of this issue it can easily hide problems.
Note: I'm not talking about file permissions but what the Jenkins webserver (Stapler?) considers the file as. If the file extension isn't something which Jenkins serves as plain text then this file is ignored by the TAP plugin.
To reproduce this issue use the following shell execution step:
rm -rf reports
cat > reports/demo << EOF
ok 1 demo OK
cat > reports/demo.pl << EOF
ok 1 demo.pl OK
cat > reports/demo.txt << EOF
ok 1 demo.txt OK
and enable "reports/*" for TAP results.
Then the following report looks like:
whereas there are 3 TAP result files available and considered for inclusion:
When clicking on the "demo" file name (not "view"!) in your browser it's considered as BIN file and download gets enforced. When clicking on demo.pl and demo.txt instead they are provided as plain-text files without forcing download to view them.
It would be nice if such "binary files" could be also considered for TAP reports or if not at least warn in the TAP execution run and mention in the documentation that files should be preferably have suffix .tap.