The 'text/html' format means HTML with text alternative and not HTML and Text. Using that format you have to create a text alternative message, usually a plain text copy of your HTML message and then paste the HTML code below. The text alternative will be shown only if recipient mail reader doesn't support HTML rendering else the HTML code is rendered. If you need to add text to your message just do it inside the HTML code.

The Text|HTML format is made of 3 parts:

1. An error message in case the recipient mail reader application doesn't understand the MIME format (automatically added). It says:

This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.

2. The text version of your HTML message in case the recipient mail reader application understand the MIME format but is not able to render HTML.

3. the HTML itself. This format will be shown when the recipient mail reader application understand the MIME format and is able to render HTML.

Note, MIME stands for 'Multipurpose Internet Mail Extensions'. It is a standard that describes how internet messages have to formated. (RFC2045).