After we moved from KMimeTypeTrader to KPluginLoader (in 16.04), we lost the ability to open mimetypes that are not directly registered with Ark, but are children of mimetypes supported by Ark.
For example the mimetypes of .odt and .epub files are not hardcoded in Ark (while application/x-java-archive is), yet they are children of application/zip and can be opened by our zip plugins.
BUG: 363717
FIXED-IN: 16.04.2