Within Mailify, you have two forms of creating your email: HTML + Text (multipart) or Text Only:
If you select "Text only" you are unable to add images to your email, nor format the text (bold, italics, size etc.) because this is done through HTML.
When you select "HTML + Text (multipart)", it allows you to format your email´s text and images. Some professional messaging services or cell phone messaging services only show the text version, which is why sending both text and HTML is encouraged, as it optimises the campaign´s deliverability.
To makes things easier, Mailify automatically generates the text format when you import/create an HTML message. Of course, you may want to edit the text version separately to account for the different method of displaying content.