mime-support turns /usr/share/applications/netsurf-gtk.desktop into
entries in /etc/mailcap like

    text/html; netsurf-gtk %s; test=test -n "$DISPLAY"

but they don't work since netsurf requires a url on the command line,
not a filename.

I see bug 577136 asks for filenames on the command line, but unless or
until that's done then perhaps a debian/mime file (to become
/usr/lib/mime/packages/netsurf-gtk) like below.  It uses liburi-perl for
filename->uri, which would create a dependency on that package.
Or perhaps there's an easier way.

I wrote:

Oops, the %s should be quoted.  run-mailcap defends against shell chars
in the filename but not against spaces.