MirrorHeadersRoot defines the root directory where to store headers for the mirrored documents.
When MirrorHeadersRoot is set to a non-empty value, indexer stores all HTTP headers or the mirrored documents to the local disk.
The default value is empty string, which means that the headers are not stored to the disk when mirroring.
Note: If you are using mirroring for crawler cache purposes and don't define MirrorHeadersRoot, then indexer will attempt to detect Content-Type of the mirrored documents according to AddType settings.