You could store the last used export path including filename in a field?
If the file is the only file in the folder you can import it to a container as a reference and then get the file name form the container attributes and use in a variable for your attachment. A bit of work.
Maybe someone has a better idea.
Wild card is not allowed. Why don't you use $filepath in Send Mail step?
$filepath in the SendMail is the easy thing to do. I guess I assumed you were not doing that already because the export and the email were not done at the same time. Maybe you did not know you could use a variable in the filepath, but it is there in the spec window as a valid example option.