there is no way to add the attatchment this way, ie this will not work:
Like <A HREF="mailto:me@mine.com?attach:afile.txt">
//Something like this could work:
<FORM ACTION="mailto:someone@somewhere.com?subject=REPORT"
METHOD="POST" ENCTYPE="multipart/form-data">
<INPUT TYPE="FILE" NAME="myReportFile" SIZE="70"><BR>
<INPUT TYPE="SUBMIT">
</FORM>
However I do not suggest using mailto for attatchments, its ugly and i don't know how well it will fair on different browsers.
I suggest using PHP to do this,
take a look at the mail() function
there are loads of possibilities.
And here is a complete mailing class, to make it even easier for you:
http://phpmailer.sourceforge.net