In order to obtain the attachments, use Get Issue Attachments activity.
Loop through the resulted array of attachment objects using Foreach activity with item of type Attachment. The “Content” property of attachment object contains the Url from where the attachment can be downloaded (if you open this URL in browser, it will automatically downlod the file).
In the workflow you can either use some VB code to download the file from URL https://stackoverflow.com/questions/36856115/download-file-from-url-code-vb-net or just use Open Browser Activity with your Jira Url, login, and then navigate to that attachment URL, and use UI automation to click download.
I hope this is helpful.