How can I use xmlstarlet to query a tag for two attribute values?

Example:

I'd like to select @id="cover" and media-type="image/*" and retrieve @href. My current "solution" seems to be...

xmlstarlet sel -t -m "//_:item[@id='$opf_cover_name']" -v @href -o '|' -v @media-type -n file.xml

...followed by some extremely messy splitting around the '|' character of the output. Does xmlstarlet have a substring function?

$ xmlstarlet --version
Version = 1.6.1
compiled against libxml2 2.9.4, linked with 20904
compiled against libxslt 1.1.29, linked with 10132