We are using Amazon S3 in order to store attachments on support cases in Salesforce. On our case layout page we have an S3 attachments section. From there you can upload files and make them internally facing, or make them externally visible to the customer depending on who the audience is.
Currently adding attachments to the support case using S3, does not result in the Attachment being copied over to the linked Jira. So the ideal behaviour would be if those attachments were available in the Jira when added to the Salesforce case.