Sending and attaching files on UNIX-like system can be done in a myriad of ways. It really depends on the mail client you are using and how you are using it.

In web-based clients like Netscape browsers, you simple click and add the attachment from the browser GUI.

The same is true for almost all GUI-based email clients.

From the command line, elm(), sendmail() and a host of others allow you to easily send files from the command line.

Most editors, vi(), et. al. allow you to read in the file, but you might want to convert to an ASCII friendly format first. There are many tools to do this. The old ones had names like uuencode() and uudecode(). Now, there are so many it is hard to keep track of them and there are used in all types of MIME setups.

To answer your question specifically, you need to specify the exact email client you are using. WWW-based email clients are popular these days, of course; but for auto-generated mail responders, etc. a command line tool is best.