Yes the txt file extension is the problem here. The standard GSEA software is written to handle this extra extension but unfortunately that protection is not present in the ssGSEA implementation on GenePattern.
In order to show and remove this extra file extension you'll need to enable your operating system's option to "show hidden file extensions". If you're unable to locate this option, let me know what operating system you're using and I should be able to help direct you.
-Anthony
Anthony S. Castanza, PhD
Curator, Molecular Signatures Database
Mesirov Lab, Department of Medicine
University of California, San Diego