The files can be accessed from every artifact, such as .bpel, .mplan, .task, .xsl, .wsdl etc., of the composite. 'oramds' protocol is used to access these files.
You need to setup your adf-config.xml file in your dev environment or Jdeveloper to access these files from MDS.
Here is the sample adf-config.xml.

xmlns:sec="http://xmlns.oracle.com/adf/security/config">












name="jdbc-url"/>





name="metadata-path"/>








credentialStoreLocation="../../src/META-INF/jps-config.xml"/>

This adf-config.xml is located in directory named .adf/META-INF, which is in the application home of your project. Application home is the directory where .jws file of you application exists.
Other than setting this file, you need not make any other changes in your project or composite to access MDS.

After setting this up, you can create a new SOA-MDS connection in your Jdev. This enables you to have a resource pallet in which you can browse and choose the required file from MDS.