There is only one file type that uses the .MNO extension:
Macromedia Design Note
Text file that contains information about a Web document; saved in an XML format and has a main <info> tag that contains descriptive items within <infoitem> tags; may be created by either Adobe Dreamweaver or Contribute.
MNO files have the same filename as the file they describe, with the .mno extension attached. For example, the file home.html may have an associated Design Note named home.html.mno.
Design Notes may include information such as the file author, status, file type, document name, and a description of the file. They may also contain other user-defined data.
Updated: February 22, 2019