Display of HTML attachments (sales receipts etc)

Attached HTML documents are not displayed as such (i.e. opening in a web browser) - they just arrive as a load of ‘jibberish’ withing the body of the email. If I send the same message (with an HTML attachment - e.g. the copy of a receipt) to Windows Live Mail, the attachment opens up correctly. (It doesn’t display as a load of jibberish in the body of the email.)

How can I fix eM Client to display the HTML attachments correctly?  (HTML images not sent as an attachment display correctly.)