Customizing the email notification that is sent by JIRA

Until today when JIRA sends a notification to a user it is in the following format:
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Key: WLBXSR-397
URL: browse/WLBXSR-397
Project: WLBX Service Requests
Type: Change Request

Reporter: Cindy Ann Honess
Assigned to: Ian Albuquerque
Attachments: DOC035.PDF
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However recently there has been a need for a custom field called 'Severity' to be included in the email notification as well:
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Key: WLBXSR-397
URL: browse/WLBXSR-397
Project: WLBX Service Requests
Type: Change Request

Reporter: Cindy Ann Honess
Assigned to: Ian Albuquerque
Attachments: DOC035.PDF
Severity 3-Medium
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How do I do it?