Post type dropdown (select multiple)

Hi,

We already have a version of the search including the post types filter which contains checkboxes (allows to select multiple) but we are now building an ‘advanced search’ with a slightly different layout. Is it possible to enable a multiple-select dropdown for the post types, similar to what we’re using with the taxonomy terms in the filters? Currently, you can only select all post types or choose individual post types to filter, but not combine them.

Original search form: ispnlibrary.org/search/

Thanks,

Peter