There is only one file type that uses the .XHTM extension:
Extensible Hypertext Markup Language Document
Web page file written in XHTML, a reformulation of HTML 4 that uses XML 1.0; includes standard HTML tags, which must adhere to more strict XML guidelines; used for publishing website content.
XHTML files more commonly use the .XHTML extension.
Updated: August 18, 2010