MimePlugin: Adding loadable fileext for Open dialog

[Florianx]

Hi,
I think, it would be a good idea to add a function to the MimePlugin, thru which a file extension could be added to the list of the "Open File"-dialog.
If you add a plugin for a new file-typ with the current Plugin class, the user can't see that files, which could loaded with it, in the open dialog. (without switch to "all files *.*").

[rickg22]

Man, we need to get all the feature requests at sourceforge organized...