Can't import Xray projects containing attachments with Jira's Project Import tool

When importing an Xray project containing attachments with Jira's Project Import tool, the import fails and the following error is triggered:

The error does not occur when importing Xray projects that do not contain attachments, as those projects are successfully migrated. Moreover, the error also does not happen when importing non-Xray projects, with or without attachments. It affects only Xray projects with attachments.

This bug affects Jira version 9.12.