Hello I have the same problem. I have form from which you can send email with attachment. When I send it without attachment everything works but when I add one the headers are encoded correctly but the email is screwed. It arrives like so:
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="==Multipart_Boundary_x6dd57d3a9e00235dcd8208baf5d169f9x"

This is a multi-part message in MIME format.

--==Multipart_Boundary_x6dd57d3a9e00235dcd8208baf5d169f9x
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit

Guru Pitka
Mariska Hargitay

--==Multipart_Boundary_x6dd57d3a9e00235dcd8208baf5d169f9x
Content-Type: image/jpeg;
name="=?utf-8?B?ZnJhdS3FocSbxI3FmcW+w73DocOtw6kuanBn?="
Content-Transfer-Encoding: base64

/9j/4AAQSkZJRgABAgAAZABkAAD//gASQWRvYmUgSW1hZ2VSZWFkef/sABFEdWNreQABAAQAAAA8
AAD/7gAOQWRvYmUAZMAAAAAB/9sAhAAGBAQEBQQGBQUGCQYFBgkLCAYGCAsMCgoLCgoMEAwMDAwM
DBAMDg8QDw4MExMUFBMTHBsbGxwfHx8fHx8fHx8fAQcHBw0MDRgQEBgaFREVGh8fHx8fHx8fHx8f
.....
My code is:

<?php
// Read POST request params into global vars
$to      = $_POST['to'];
$from    = $_POST['from'];
$subject = $_POST['subject'];
$message = $_POST['message'];
$subject = "=?utf-8?B?" . base64_encode($subject) . "?=";

// Obtain file upload vars
$fileatt      = $_FILES['fileatt']['tmp_name'];
$fileatt_type = $_FILES['fileatt']['type'];
$fileatt_name = $_FILES['fileatt']['name'];
$fileatt_name = "=?utf-8?B?" . base64_encode($fileatt_name) . "?=";

if (ereg("(.*)<(.*)>", $from, $regs)) {
   // There is a name for the expeditor !
   $from = '=?UTF-8?B?'.base64_encode($regs[1]).'?= <'.$regs[2].'>';
} else {
   // Nothing to do, the from is directly the email !
   $from = $from;
}

$headers = "Content-type: text/plain; charset=UTF-8\r\n";
$headers .= 'From: '.$from.chr(10);

if (is_uploaded_file($fileatt)) {
  // Read the file to be attached ('rb' = read binary)
  $file = fopen($fileatt,'rb');
  $data = fread($file,filesize($fileatt));
  fclose($file);

  // Generate a boundary string
  $semi_rand = md5(time());
  $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
  
  // Add the headers for a file attachment
  $headers .= "\nMIME-Version: 1.0\n" .
              "Content-Type: multipart/mixed;\n" .
              " boundary="{$mime_boundary}"";

  // Add a multipart boundary above the plain message
  $message = "This is a multi-part message in MIME format.\n\n" .
             "--{$mime_boundary}\n" .
             "Content-Type: text/plain; charset="UTF-8"\n" .
             "Content-Transfer-Encoding: 8bit\n\n" .
             $message . "\n\n";

  // Base64 encode the file data
  $data = chunk_split(base64_encode($data));

  // Add file attachment to the message
  $message .= "--{$mime_boundary}\n" .
              "Content-Type: {$fileatt_type};\n" .
              " name="{$fileatt_name}"\n" .
              //"Content-Disposition: attachment;\n" .
              //" filename="{$fileatt_name}"\n" .
              "Content-Transfer-Encoding: base64\n\n" .
              $data . "\n\n" .
              "--{$mime_boundary}--\n";
} else {
  $headers = "Content-type: text/plain; charset=UTF-8\r\n" .
  $headers = 'From: '.$from.chr(10);
}


// Send the message
$ok = @mail($to, $subject, $message, $headers);
if ($ok) {
  echo "Email sent!";
} else {
  echo "Error occured!";
}
?>

Can you tell me what I am doing wrong.