If you want a quick solution, you could select the option to export all files instead of only selected resources.
Or you could add *.bme to the "Filter to export non-resource files" field.
I don't know if the fact that they are not displayed in the resources list is a bug, or if it's because it displays only resources, i.e. files which are linked to nodes in a way that the editor knows about. Do you see them in the Filesystem dock?