When pasting into ATTO, class names are stripped out

When a page HTML contains a class on a <p> <table> etc, pasting plain text from notepad etc into the ATTO editor, causes all the class names to be stripped out of the html.

<p class="myClassName">Lorem dolar sit amet</p>

Then paste plain text using WYSIWYG. Looking at the HTML now shows the class was stripped from the <p> tag.

<p>Lorem dolar sit amet</p>

<p>New pasted text</p>