amd64\ttyui.hlp

WinHelp file Reference

A .hlp file contains help content in the WinHelp format. The binary file holds topics, images, and links for the classic Windows help viewer. Microsoft replaced the format with Compiled HTML Help, .chm, in 1997. Windows Vista and later versions do not ship the viewer any more. Old applications and drivers still carry .hlp files.

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.