@Override
    public String toString() {
        return "FieldNameMappingFilter{" + "mappings=" + mappings + ", excludeUnmapped=" + excludeUnmapped + '}';
    }
If this is <code>true</code> (default), this means that only the fields that
have a "from" value in the mapper will be passed through.  Otherwise,
this will pass through all keys/values and mutate the keys
that exist in the mappings.

@param excludeUnmapped
tika-core/src/main/java/org/apache/tika/metadata/filter/FieldNameMappingFilter.java