There is only one file type that uses the .MAILSTATIONERY extension:
Apple Mail Stationery File
Email stationery file created and used by Mail, an email application bundled with OS X; contains files that make up a stationery design, such as .JPG, .PNG, and .HTML files, along with .STRINGS files and a .PLIST file that describe the design.
Mail comes with a variety of default MAILSTATIONERY files, each containing a design you can insert into your email message. To insert a stationery design, select the "Stationery" icon in the top-right of the message window and select the design. You can also create your own custom stationery design by opening a new message, creating your design, and then selecting
Updated: October 31, 2014