8.5.1 File Data Source Directives
This section of the Configuration File defines the mapping of the file extension to the MIME content type of the data in the file. The key will specify the file extension and the value will specify the MIME type / Content type to be used for the transaction.
Example
xml=text/xml
txt=text/plain
defines that the files with the extension .xml will be sent as content type text/xml and files with the extension .txt will be sent to the host as content type text/plain.