Quoted from Meanie's great websiteFile Associations
The file association information is derived from two places. The file /home/QtPalmtop/etc/mime.types stores the mime types that is used to derive the association information which is then combined with the desktop files stored under /home/QtPalmtop/apps
The information inside mime.types specifies what file extensions are associated with each file type. The desktop file for each application then specifies which file type the application should handle. If you have more than one application associated with a certain file type, then only one of them will be associated, however, the logic that the Z uses to determine which one to use is unknown to me. Therefore, make sure that only one of them is associated to a file type and you will get the expected file association.
As an example, to associate Opera to .htm and .html files, you would have the following in mime.types
text/html html htm
And opera.desktop would have the following:
You can also associate an icon to the file association as well which is shown in the above sample. Multiple associates can be delimited with a semi colon (