Unable to add .ics (calendar) and .msg (outlook) file as autoresponder attachment

I'm trying to 

I go to Advanced and select Add Attachment, Upload File, change Customised Files to All Files and select the .ics file. I then receive the error message Unsupported file type (see below). I am able to upload other types of files such as jpgs, pngs, pdfs. But I can't upload any type of Outlook file e.g. ics or msg.

Is this a setting I can change?

Thanks for reporting. I was able to reproduce it here. It's probably not yet added as supported file type in the autoresponder attachment. 

I have escalated this as a feature request. You will be informed here once this is implemented.