If you are looking at the file in the Windows Explorer (files and folders) then you probably have the suffixes for known filetypes hidden. It will then default to using a description from Windows, which is already used for whatever a Certificate Trust List is. See this page: http://en.wikipedia.org/wiki/STL_%28file_format%29#Notes
Anyway, this description is based on a setting used by Windows Explorer to determine the application to use if you double click on a file. Like .txt. files will open with Notepad and .doc files will open with Word, regardless of the actual content of the file.
Two things you might do - one is to change the association that Windows Explorer has by selecting the file, right clicking and picking Open With. The select Choose Default Program and pick one that appeals to you, such as the software you will use to run the printer. This really makes no difference for any file you would open from within a program, it is only a guide to Windows Explorer.
The other thing you might do is to select the Windows Explorer menu (hit Windows Key + E) (WIn 7, other versions are a little different) named "Organize" and chose "Folder and Search Options". It will open a new window called Folder options. On the tab named View is an entry called "Hide extensions for known file types." Unselect this to see what the actual file extension is.
Neither one of these changes is required, they are convenient - the software for the printer does not depend on the artificial file type that Windows Explorer thinks a file has. It only matters if you intend double clicking on a file and want something to happen as a result.