psreadline.format.ps1xml

PowerShell format and type definition Reference

A .ps1xml file contains formatting or type definitions for PowerShell in XML. Format files control how PowerShell shows objects, for example as tables or lists. Type files add members to .NET types inside PowerShell. Modules load these files when they load. The content is XML, thus a text editor can show it.

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.