mno is a file extension created by some web designing software like UltraDev or Dreamweaver. It often goes after the existing extension of the webpage (.html, .asp, .php), in effect creating a double extension, for instance, filename.html.mno.

When a page with a recordset is created by some web design software, an .mno file is created in the site's _notes folder. .mno file contains design notes. This folder is usually not visible in the site manager, but it can be viewed using your system's file management tool. The MNO file contains XML information that are used to gain information about recordsets on the page. The XML file defines the name and data type of the recordset columns. Web Designing software need this file to properly populate the data bindings panel in their design environment.

Read more about Mno:  Potential Issue With Missing MNO File