The file extension MNO is used for so-called Design Notes (still stands for Macromedia) of the web page editor Adobe Dreamweaver. Usually, the ending MNO is used as a double ending, that is attached to an existing extension, for example bild.Jpg.Mno.
These files contain additional information about the associated file, such as comments, the status of the file, and the link to the original file. In addition, Dreamweaver uses Design Notes for integration with Adobe FireWorks and Flash, so when you edit an image file, it automatically opens the right program and the changes you make will immediately affect the page.
If you do not want to use the Design Notes, you can disable this feature in the properties of the site, 'Site-> Manage Sites-> Edit' under the 'Advanced' tab and the 'Design Notes' category.