Solution
If you look at the content being sent in the comment containing attachments - it contains something like
By converting it into something like
!https://d3v-francis.zendesk.com/attachments/token/AzEkdy3kmZ8X9MsqzAlG8Aarg/?name=hpalm-jira.png!
Jira will be able to show the inline attachment.
Note that works only if the attachments is accessible under that token. The day that the token is invalidated, the attachment will not be reachable, so this is not really a solution.
Implementation
It is easy to replace the  into !<filenname>! by using following construct
Line 1 uses the mergeComments to iterate over all the comments in the replica and summons the closure to be run.
Line 4 calls the replaceAll groovy string method. For each string segment matching the regexp a replacement is created. The replacement contains the necessary code to show the image with a maximum width of 512.
Code is available here
Per the title, my use case is for syncing data between Zendesk and Jira.
I have gotten description formatting to sync using:
and comment formatting to sync using a custom script rule.
def includeComments = commentHelper.mergeComments(Jira.issue, replica).collect{it} Jira.issue.comments += nodeHelper.toMarkDownComments(includeComments)However, any in-line image in a note in Zendesk results in a Jira comment that looks like:
Exalate support had asked me (In a zoom call) to post a question over here to document how to use the html to wiki feature in the script.