Problem

When user receive notification from JIRA, the notification contains attachment like ATT00001.htm, ATT00002.htm and so on)

Diagnosis

• Looking into the notification that contains the unknown attachment, only notification with known attachment contains the additional unknown attachment.

Cause

The unknown attachment is added because the attachment body part in an e-mail message is sent before the message body part of the e-mail message is sent. This is mentioned in the Microsoft KB below:

• https://support.microsoft.com/en-us/help/969854/the-body-of-a-message-is-shown-incorrectly-as-an-attachment-if-you-try-to-use-an-application-in-an-exchange-server-environment-to-send-a-message-that-includes-attachments
  
  and
• http://kb.mit.edu/confluence/pages/viewpage.action?pageId=4981187

Workaround

1. Reorder the order of the body sent by following instruction from Microsoft KB : https://support.microsoft.com/en-us/help/969854/the-body-of-a-message-is-shown-incorrectly-as-an-attachment-if-you-try-to-use-an-application-in-an-exchange-server-environment-to-send-a-message-that-includes-attachments
2. If the solution from KB is not application (eg: if the mail server is Office 365 based), user can try different Mail Server which is POSTFIX based.