Some Mac files have a "Creator" and "Type" identifier rather than a file extension. This comes from Mac OS 9 and previous versions of the Mac OS, in which most Macintosh files did not have file extensions. In Mac OS X, nearly all filenames include an extension, though files with no extension can still be opened.

If the associated program for the "extensionless" file is installed, Mac OS X should automatically open the file with the correct program. If the application is not available, you may get an error saying the file cannot be opened. If this is the case, you may try to drag the file to an application you think might open the file. For example, if the file is a text file, try dragging it to TextEdit. If you think the file is a picture, try opening it in Preview. If it is an audio or video clip, try dragging the file to QuickTime Player to see if it will open.

Sometimes it is difficult to determine the file type of a file with no extension, but there are some programs that will show you the Creator and Type information, which can be helpful. For example, if the Creator is listed as "SIT!," you may be able to guess that the file is a StuffIt file, and therefore can be opened with StuffIt Expander.

Two programs that allow you to view the Type and Creator information of files include NameCleaner and FileType. These programs also let you alter the Creator and Type information, which usually should not be changed. If the Creator and Type information is modified incorrectly, no programs will recognize the file.

If you have tried the steps above and still cannot open the file, it is possible the file belongs to a Windows program and is just missing a file extension. You may want to transfer the file to a Windows-based computer to see if a Windows program will open it.