When using high definition PDF (200 dpi or more), then preview pdf from the attachments macro doesn't work and just show the PDF as blank.

This bug is related to the older bug regarding PDF preview opening in blank:
https://jira.atlassian.com/browse/CONF-31759

The old bug was closed as obsolete since we have a new preview macro for PDF that doesn't cause this issue when opening the PDF files, however it seems that the attachments macro still uses the old macro.

Workaround:
Use the new file macro instead of the attachments macro or disable the preview in the attachments macro options.

possible FIX:
Changing the old attachment macro code to embed the new file and attachments viewer macro from 5.7 will fix this issue.