@Field
    public void setOnExists(String onExists) {
        switch (onExists) {
            case "skip":
                this.onExists = ON_EXISTS.SKIP;
                break;
            case "replace":
                this.onExists = ON_EXISTS.REPLACE;
                break;
            case "exception":
                this.onExists = ON_EXISTS.EXCEPTION;
                break;
            default:
                throw new IllegalArgumentException("Don't understand '" + onExists + "'; must be one of: 'skip', 'replace', 'exception'");
        }
    }
What to do if the target file already exists.  NOTE: if more than one
thread is trying write to the same file and {@link ON_EXISTS#REPLACE} is chosen,
you still might get a {@link FileAlreadyExistsException}.

@param onExists
tika-pipes/tika-emitters/tika-emitter-fs/src/main/java/org/apache/tika/pipes/emitter/fs/FileSystemEmitter.java