Add mailcap
    
    /etc/mime.types is used as a file dependency in the buildroot (e.g.
    httpd-core, for git).  Building mailcap in /app for flatpaks (namely,
    for perl-LWP-MediaTypes as a dependency of perl-libwww-perl) breaks
    the buildroot in that case.  Instead, perl-LWP-MediaTypes will need to
    use the fixed location rather than %_sysconfdir.