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.
Most often resolving problems with opening MNO files is very simple. Just install an appropriate program that supports such files. All of the listed programs support MNO files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of MNO files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate MNO files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Adobe Contribute applications is not on the list, select "Browse" option in order to manually locate the directory where Adobe Contribute has been installed.
Sometimes problems with opening MNO files may not lie with the application itself, but can arise due to other causes, such as: