You may use the following procedure to whitelist certain attachments through the Exim filter.
This article is based on the following documentation:
How to customize the Exim filter
1. As the root user, login to the server via SSH or use the Terminal feature in WHM
2. Make a copy of the attachment filter:
cp -v /usr/local/cpanel/etc/exim/sysfilter/options/attachments /usr/local/cpanel/etc/exim/sysfilter/options/attachments_custom
2. Open your newly created custom file in the text editor of your choice. Usually either nano or vim:
3. Examine the filter closely and edit the regular expression so that it no longer matches the attachment that you would like to whitelist. A simple grep for your attachment won't work a lot of the time due to the nature of regular expressions. If you make an edit that breaks the filter, you can copy the original attachments filter to your custom filter to start over fresh.
4. Login to WHM as the root user
5. Navigate to: Home »Service Configuration »Exim Configuration Manager
6. Click on the "Filter" tab
7. Disable the "Attachments: Filter messages with dangerous attachments" option
8. Enable your custom attachments filter option
9. Click on the blue save button at the bottom