Email messages can be sent / read in either plain text or HMTL format.
You might have chosen to send in html format, but read in plain text format.
When you read in plain text format, the received email, if sent in html format, will be made as an additional attachment.
It's normal. To read in plain text is actually a safe setting.
But you can have a look at and change your settings if you like, by clicking Tools, Options, the Read and Send tabs.