Different plugins can have different metadata/properties for the same mimetype.
To handle that, we create an ArchiveFormat instance out of the preferred plugin for a given mimetype.
For now only the properties about encryption are set in the json. The rationale is the following: if a json doesn't have a given key, a default value is assumed. For instance, the libarchive plugin doesn't have any Encryption or HeaderEncryption key ==> the ArchiveFormat will have the Unencrypted property set.
There is currently some code duplication with the Archive class. Eventually the Archive class will use these new functions in mimetypes.cpp, to load the plugins.