Reasons:
A media type is type/subtype, not major/minor, and while white space is technically allowed after the slash, it is rather uncommon to have it there.

Corrected text:

install
    Adds the file type descriptions provided in mimetypes-file to the desktop
    environment. mimetypes-file must be a XML file that follows the
    freedesktop.org Shared MIME-info Database specification and that has a
    mime-info element as its document root. For each new file type one or more
    icons with name type-subtype must be installed with the xdg-icon-resource
    command in the mimetypes context. For example, the filetype 
              application/vnd.oasis.opendocument.text 
    requires an icon named 
              application-vnd.oasis.opendocument.text 
    to be installed (unless the file type recommends another icon name).