When you click Add on the page, the Add File Type Group Entry dialog box is displayed. You have to specify a MIME type, a file type, and a file type configuration for the entry.
When the value of the File Type selection for a MIME type is “Not Translatable”, the “File Type Configuration” column is blank for that entry. If the file type resolution process finds that a MIME type is set to “Not Translatable,” it treats assets of that MIME type as not translatable.