Corrupted Attachments ???
I spent many hours with corrupted attachments (of all types of files) - The answer: a blank line is needed after $msg.=$file \r\n \r\n [incredible but true].
Heres some useful code for sending an attachment, and display html OR text depending on the users email-reader.

i work with many different systems, so...

<?php # Is the OS Windows or Mac or Linux
if (strtoupper(substr(PHP_OS,0,3)=='WIN')) {
  $eol="\r\n";
} elseif (strtoupper(substr(PHP_OS,0,3)=='MAC')) {
  $eol="\r"; 
} else {
  $eol="\n"; 
} ?>

<?php
# File for Attachment
$f_name="../../letters/".$letter;    // use relative path OR ELSE big headaches. $letter is my file for attaching.
$handle=fopen($f_name, 'rb');
$f_contents=fread($handle, filesize($f_name));
$f_contents=chunk_split(base64_encode($f_contents));    //Encode The Data For Transition using base64_encode();
$f_type=filetype($f_name);
fclose($handle);
# To Email Address
$emailaddress="user@example.com";
# Message Subject
$emailsubject="Heres An Email with a PDF".date("Y/m/d H:i:s");
# Message Body
ob_start();
  require("emailbody.php");     // i made a simple & pretty page for showing in the email
$body=ob_get_contents(); ob_end_clean();

# Common Headers
$headers .= 'From: Jonny <jon@example.com>'.$eol;
$headers .= 'Reply-To: Jonny <jon@example.com>'.$eol; 
$headers .= 'Return-Path: Jonny <jon@example.com>'.$eol;     // these two to set reply address
$headers .= "Message-ID:<".$now." TheSystem@".$_SERVER['SERVER_NAME'].">".$eol;
$headers .= "X-Mailer: PHP v".phpversion().$eol;           // These two to help avoid spam-filters
# Boundry for marking the split & Multitype Headers
$mime_boundary=md5(time());
$headers .= 'MIME-Version: 1.0'.$eol; 
$headers .= "Content-Type: multipart/related; boundary=\"".$mime_boundary."\"".$eol; 
$msg = "";

# Attachment
$msg .= "--".$mime_boundary.$eol;
$msg .= "Content-Type: application/pdf; name=\"".$letter."\"".$eol;   // sometimes i have to send MS Word, use 'msword' instead of 'pdf'
$msg .= "Content-Transfer-Encoding: base64".$eol;
$msg .= "Content-Disposition: attachment; filename=\"".$letter."\"".$eol.$eol; // !! This line needs TWO end of lines !! IMPORTANT !!
$msg .= $f_contents.$eol.$eol;
# Setup for text OR html
$msg .= "Content-Type: multipart/alternative".$eol; 

# Text Version
$msg .= "--".$mime_boundary.$eol;
$msg .= "Content-Type: text/plain; charset=iso-8859-1".$eol;
$msg .= "Content-Transfer-Encoding: 8bit".$eol;
$msg .= "This is a multi-part message in MIME format.".$eol;
$msg .= "If you are reading this, please update your email-reading-software.".$eol;
$msg .= "+ + Text Only Email from Genius Jon + +".$eol.$eol;

# HTML Version
$msg .= "--".$mime_boundary.$eol;
$msg .= "Content-Type: text/html; charset=iso-8859-1".$eol;
$msg .= "Content-Transfer-Encoding: 8bit".$eol;
$msg .= $body.$eol.$eol;

# Finished
$msg .= "--".$mime_boundary."--".$eol.$eol;   // finish with two eol's for better security. see Injection.

# SEND THE EMAIL
ini_set(sendmail_from,'from@example.com');  // the INI lines are to force the From Address to be used !
  mail($emailaddress, $emailsubject, $msg, $headers); 
ini_restore(sendmail_from);
?>


I hope this helps.
Jon Webb [Madrid&London]

[EDITOR thiago NOTE: Code has a fix from o.straesser AT gmx DOT de]