Comment 38 for bug 877256

kdelib5-data defines the mimetype as:

" <mime-type type="application/x-webarchive">
    <sub-class-of type="application/x-compressed-tar"/>
    <sub-class-of type="application/x-gzip"/> <!-- only necessary because x-compressed-tar is not specifying this sub-class-of (shared-mime-info 0.8) -->
    <comment>web archive</comment>
    <glob pattern="*.war"/>
  </mime-type>"

it's a subtype of zip but that doesn't seem to be enough to make file-roller happy