I'm able to successfully monitor a log file on a Windows server (2008 R2) using the Universal Forwarder while on version 4.3.1. The entry in inputs.conf is a simple [monitor://<path to file>], no additional options are used.
I performed an in place upgrade to UF 6.0.2 and I don't get anything from that file indexed anymore. I still get event log entries, it's just that specific file that is not being indexed.
splunkd.log on the host shows the file is being monitored as I see the TailingProcessor entries mentioning the stanza. splunk list monitor shows the file is being monitored.