Also, it is up on your server to allow larger attachments, check your settings for this. If you upload an attachment larger that your limits, it will be discarded.
The way we handle attachments is to have a folder with the attachments inside and then have a simple link to it in the email. No problem with mwz as no attachment is sent or uploaded via mwz.Looks like is happening again, worked for an instance and now doesn't work, even with the file extension and mime types added, it sends all the listed formats but the one's I added in this case, two important ones. (jpg and png).