There is a Github script available for this (https://github.com/RallyTools/Rally-Export-Attachments).
Please note, that CA Agile Central support does not actively maintain or support Github tools and scripts. If you have a problem or question, we recommend filing a Github Issue or posting a question to Stack Overflow: http://stackoverflow.com/questions/ask?tags=rally.
Another option is to engage Technical Services to assist, this will be a fee based effort, the fee would be based on the amount of artifacts and the magnitude of the export. contact the account team or Technical Services to initiate this effort.