There is no explicit monitoring of a single file. To apply protection/monitoring to the file you need to apply a tag or a category with the appropriate tagging or categorization rules. If the specific file you are looking to monitor is stored on a file share, you can use a Registered Document Repository to fingerprint the file and its content and apply a Content Category. Otherwise, a combination of text patterns and dictionarys will be needed.
Thanks dgriner. I've spent some time with DLP and I've now succesfully created a classification rule by identifying specific text strings combined with the file type and extension.
I've applied this to a monitoring protection rule for Email and Network, and this seems to be working fine.