There has been a report of .ics files not working as suggested. They are a supported file type and they should be able to be added to an email blast.
New update - even if you can upload these files to emails we do not allow you to link to them as these are ways that malware is spread and your emails will be very likely to get caught in spam traps. At the moment we recommend linking to these files in Dropbox or Google Drive.
Update - we are unable to successfully attach an .ics file to an email - in the meantime it is best to link to the file uploaded somewhere else.
We need to do some testing to further confirm this bug.