@Field
    public void setCommitWithin(int commitWithin) {
        this.commitWithin = commitWithin;
    }
Options: SKIP, CONCATENATE_CONTENT, PARENT_CHILD. Default is "PARENT_CHILD".
If set to "SKIP", this will index only the main file and ignore all info
in the attachments.  If set to "CONCATENATE_CONTENT", this will concatenate the
content extracted from the attachments into the main document and
then index the main document with the concatenated content _and_ the
main document's metadata (metadata from attachments will be thrown away).
If set to "PARENT_CHILD", this will index the attachments as children
of the parent document via Solr's parent-child relationship.
tika-pipes/tika-emitters/tika-emitter-solr/src/main/java/org/apache/tika/pipes/emitter/solr/SolrEmitter.java