If you want your operation to return a text document that contains something beside HTML you might want to set the MIME type in the operations definition. Setting the MIME type in the operation definition signals the Product Server that the output content will be something other than HTML and it will insure that the ContentType HTTP header is set to the MIME type you specified.
For example, suppose your operation will return a KML file to be used by Google Earth. If your operation definition looks like this:
<operation ID="MakeKML" output_template="create_kml" mime_type="application/vnd.google-earth.kml+xml" ... >
then LAS will set the ContentType header to the value you specified. In this case the MIME type that is appropriate for KML files. Then if the client making the request is a browser, it will know that the response should be handed off to the Google Earth application. If the browser is properly configured then Google Earth will open automatically when this response comes back from LAS.