Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window corda-ledger-extensions-fungible.corda.cpk-0.9.0-Iguana1.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.