The enabledPlugin property is a reference to a Plugin object that represents the installed and enabled plug-in that handles the specified MIME type. If the MIME type is not handled by any plug-ins, then the value of this property will be null.
The navigator.mimeType array will tell you whether a given MIME type is supported by the browser in some fashion. The enabledPlugin property of the MimeType object, however tells you whether a particular supported type is supported with a plug-in or not (MIME types can also be supported with helper applications, or directly by the browser.) If a MIME type is supported by a plug-in, it can be embedded in a web page with the <EMBED> tag; otherwise it must be output in some other way.