If it's your first Google Code-In task it's highly recommended to start with Become a community member.
- The student will need to have a Jira account, if it's not the case you can signup here
- The student will also need a https://www.github.com account. You might also want to take a look at Git and GitHub interface documentations.
- Assign yourself to the Jira issue
The API ScriptMimeMessage#addPart should allow getting a BodyPart directly in input (which essentially means not touching it).
- The student must commit the changes and create a pull request on https://github.com/xwiki/xwiki-platform
- The summary of the Pull Request must have a first line matching the JIRA issue following by a colon and then the title of this issue
- Add a comment in the jira issue mentioning your pull request and provide a link to it