Images are not embedded inline in filter subscription emails

Icons/Images are not embedded in-line when JIRA sends filter subscription emails. This causes problems for users who access their email from outside the firewall.

Steps to reproduce
Create a filter
Add a subscription
Run the subscription

Expected results
Notification is sent displaying all images inline successfully

Actual Results
Notification is sent but the images are not inlined.
No icons will be displayed if the email client is not able to reach the JIRA server due to firewall / network restrictions