A properties file containing additional mappings for the context. They key is the class and the value is the symbol that should be used. This mapping file will be consulted for symbols for context classes before trying the internal mapping, which only covers a subset of contexts.
To find out which class you need to map create a Logger for the class io.jenkins.plugins.customizable_header.headers.ContextAwareHeader on level fine. This will print the list of contexts when loading a page, e.g. Context: hudson.logging.LogRecorder.
The path can be absolute or relative to JENKINS_HOME.