setup_help_upgrade_or_custom.rtf

Rich Text Format document Reference

An .rtf file contains a document in the Rich Text Format. The format stores text together with formatting, for example fonts, colors, and tables. The content is readable ASCII with control words. Microsoft developed the format, and most word processors can open it. WordPad on Windows used .rtf as its native format.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.