log in as host, go to host->host settings. Expand the "other settings" area, and you'll see a section called "file upload extensions", that controls what types of file can be uploaded. The default is "swf,jpg,jpeg,jpe,gif,bmp,png,doc,xls,ppt,pdf,txt,xml,xsl,css,zip". If you want to support additional ones add them to the comma delimited list.
Note: there is a checkbox during upload that allows you to ignore invalid extensions. This allows you to ensure that you only allow invalid extensions deliberately such as when you upload modules.
Cathal