I was unable to come up with a solution which would reliably create attachments in our Jira instance. So our Jira/Java developer created a custom processor/endpoint for base64-encoded attachments sent over from ServiceNow.

100 days spent on this ? wow, that is a long time.... How many user resources on that ?

As for our implementation, it was just me and the fore-mentioned Jira developer. We put together a fully working solution ( against multiple JIRA instances ) in about 4 weeks but I am sure yours must have been a bit more involved...

Cheers