Follow the instructions in the message and you're on. A complete summary of commands will be sent to you. At the time of writing, the list usually receives between 0 to 20 posts a day. There are typically 5 or 10 posts in a day. If you send a file to the mailing list, you risk flooding those mailboxes.
Some people have slow network connections.
If files begin to appear on the list, it would take these people an inordinate amount of time to download their mail. If you have a file to distribute, you should post it to your website and send the URL to the list. HTML is the language of the World Wide Web. If your mail program allows you to change the colour, font, size or any other aspect of the text you are writing, you are probably posting to the list using this document markup standard. Plain text is just that.