As the feature is new in the Mac environment, so is for you as well – you should contact an Apple Safari customer support service provider and access a reliable tech help from the experts.
The Apple iPhone doesn't offer an automatic configuration for Comcast email, due to which it makes the process of adding your account a bit more confusing.
Probably you might have experienced that when you attempt to open a file extension on Mac, it gives out an error message and does not let you open the file.