* SOLVED *
web.xml was carried over from a previous version.
there were a couple of classes that after the upgrade were mapped to the wrong folders.
fortunately the more we have to reverse engineer the abandoned Jira server, the easier it us for us to just build a replacement ourselves.
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Upon upgrade to Jira Software 9.4.0, none of the attachments in issues work.
The issue still has links to image attachments, but trying to open the link gives a 404 error. The attachments themselves seem to still exist in the Jira data folder, but the Tomcat "secure" folder doesn't seem to have an attachment link to it.
