The "System/Send Email" dialog will send email to selected JIRA users, formatted as "Text" or "HTML". If "Text" is selected (the default), what is sent will be a multi-part MIME message, with the "Content-Type" of the first part labeled as "text/plain". However, the actual content type is HTML, and is not very different (except for the "Content-Type" header) from what is sent when HTML is selected. The result is nearly unreadable in most email clients.

I suspect the bug was introduced in v6.2, and persists in 6.2.1.