What types of files can be attached to Autoresponder email?

What types of files can be attached? I’m trying to attach a voice recording to an auto response email that is a m4a file type and it will allow it. Also what is the max size for a file?

Unfortunately, the Autoresponder Upload Attachment function is very limited to some types of documents and pictures (pdf, doc, jpg, png etc). There are only shown files supported when you open a dialog window to attach a file. 

But, if you need to add some specific or "heavy" file to your Autoresponder email, I can suggest to store it at some 3rd party server and to add a link to it in the email:

Hope this information helps!