I am not familiar with Jira's web service calls, but with others that I have worked with, I have found that to send a file to another system, it needs to first be converted to Base 64.  
Here is some info from the guru that might be able to help with that.
I used this to send a file from SNow to Workday.
Sending attachments to a 3rd-party service desk - ServiceNow Guru