Messages with attachments not converted

[sjoerdHME]

After adding a attachment the mimetype of the message is replaced by 'multipart/mixed'. In the CssInlinerPlugin class I've added

if (($message->getContentType() === 'text/html') || ($message->getContentType() === 'multipart/mixed')) ```

Can I just do that ?

[hkdobrev]

I think this should be solved by the plugin by traversing the children parts of the message being sent.