It does not look like you have no other choice I can find. You may have some luck with a compression utility like these listed.
File Viewer Plus
Corel WinZip 21
Microsoft System Configuration Utility
The .XSN is a file form template used by Microsoft InfoPath, a form design and creation program; saved as a compressed .CAB (cabinet) file, which contains multiple files.
More Information on its contents are listed here.
XSN files include a schema (.XSD) file, an .XSL style sheet, .JS or .VBS script files, and image files used by the form interface.