There is only one file type that uses the .XHTML extension:
Extensible Hypertext Markup Language File
An XHTML file is a web page written in XHTML, a reformulation of HTML 4 as an XML 1.0 application. It uses standard HTML tags, but must conform to more strict XML guidelines.
Many current websites include Web pages based on either the XHTML 1.0 or XHTML 1.1 specification.
Updated: March 4, 2016