We are using the RESP API calls too. However our customers usually do not expose their JIRA instances directly to the Internet so we call the REST API through a MID server.

The attachment data should be encoded using Base64. It is not straightforward to send the attachments. We had to write a couple of Java classes as a MID server plugin to perform an HTTP POST multi-part request. Standard ServiceNow RESTMessage does not support multi-part requests.

I know it was a lot of work. We spent like 100 man-days to implement the whole JIRA integration. If anybody is interested we offer the JIRA integration as a product / package. Just send me an PM.

Pavel