Thunderbird often likes a file:// prefix for files. In other situations where a file is attached, there is a file browser and it automatically fixes up the pathname, converting it to an URI format. It's a shame this is just a text entry box that leaves you to your own devices. I do see that if I drag and drop the required file onto the dialogue, its address is reformatted. I also note that google calendar doesn't accept attachments, but my local "Home" calendar and another that uses an ics file do seem prepared to accept attachments. I doubt that the calendar itself has any tools to attach real files, but offers this compromise of storing the address of a file. Now, if I were to view my calendar on another device, the address of a file on this particular computer wouldn't be very useful. If you really want to share a file via the calendar, I think you need to store that file somewhere universally accessible such as a webserver, ftp server, dropbox, onedrive etc and then publish the link to that location as your attachment in the calendar event.