Issue Summary

As a possible workaround to use custom priority icons, you can upload the desired icon as an attachment to an issue, and use its URL in the priority configuration.

But, currently, the filter subscription email will not be able to display those icons.

Steps to Reproduce

• Upload an icon to an internal Jira issue, and copy its URL from there (you can do so in the old view);
• Create a new priority, using that URL as the icon;
• Perform an advanced search;
• Set as Priority as a column:
  
• Save the search as a filter;
• Create a filter subscription:
  
• Trigger the subscription:
  

Expected Results

The custom icon will be displayed in the filter subscription:

Actual Results

The icons appear in a broken state, or are not displayed at all:

Workaround

Upload the icon to a different online repository, and copy the URL from there.