Missing file extensions causing errors with files opening in OS X

OS X by default hides file extensions on files, so users may be confused when they have files that are not recognized at all by OS X and hence do not open.

One source of the error is the fact that the file in question has no extension at all.

To check for a single file and its missing extension:

1. Right-click a file and select Get Info. If you have a Mac with a one-button mouse, hold the “Control” key and click the file.
   #Click the triangle next to Name & Extension. The text field will display the full file name and extension.
   #Click inside the text field and use the “Delete” key to remove the original file extension.
   #Type the desired new file extension and close the window.

Source:
http://science.opposingviews.com/add-extension-file-mac-10647.html