How to ignore mime type for maff file extension?

I have this Mozilla Archive Format, with MHT and Faithful Save addon installed with Iceweasel on debian 8.3 jessie stable. The addon is useful in saving webpages into single file with extension **maff**. The mimetype for this maff file is application/zip. Because of this, whenever I open maff file, it opens in Archive Manager or analogous application. In order to open maff files in iceweasel, I have chosen Open with Iceweasel in file properties. Doing so opens maff files with iceweasel, but also opens every zip file in iceweasel with a download dialogue box. I think this is because the system has set mime type application/zip with iceweasel. All I want is to open maff files with iceweasel/firefox and open zip files in regular Archive Manager. How can I achieve this?