If you want to send multiple files at once, archive them first. In true MailBigFile style, there are 3 simple steps ...
Select all the files you want to archive.
Control-Click on the files, and select "Create Archive".
A ZIP/Archive containing all the files will be created. It is this ZIP file you send.
N.B. The recipient just needs to double click on the ZIP, and the containing files will appear!