The manpage states (under install)     
              [...]
              in the mimetypes context. For example the
              application/vnd.oasis.opendocument.text filetype requires an
              icon by the name of application-vnd.oasis.opendocument.text to
              be installed.

However, xdg-icon-resource complains if this convention is followed to the letter:

$ xdg-icon-resource install --size 32 icons/red-32.png xdgtestdata-icontest
xdg-icon-resource: icon name must have a .png, .xpm or .svg extension

Therefore the xdg-mime documentation should be updated to include statements
about the acceptable icon formats and filename extensions, or xdg-icon-resource
should be modified to install extensionless icon files.