When we load the various XMLs via the Configuration class, the source of the XML file (filename) is usually kept in the Configuration class but not exposed programmatically. It is presently exposed as comments such as "Loaded from mapred-site.xml" in the XML dump/serialization but can't be accessed otherwise (Via the Configuration API).
For debugging/etc. purposes, it may be useful to expose this safely (such as an API for "where did this property come from?" queries for a specific property, via an API.