The mail() function's job is to send email - that's it. You give it an address, subject, body, and optionally some extra headers, and it sends email
However, you can easily open a text file and read it in, mailing out the text. The easiest way to do this is to use the file() function:
$text = join (file ("/path/to/file.txt", ""
mail ("email@example.com", "Contents of file.txt", $text);
File() returns an array (split by line terminators) of the text in the file, and also works for remote URL's. Since it keeps the line terminators in tact, you can simply join it into a string and mail that out.
Hope you find this helpful