Sending PHP form even if file is not attached - php

Question

I am using the code below from a tutorial to send files attached to a form on my website. I have added some variables which were missing and the form is working fine now. But the thing is I can't sent the form WITHOUT a file attached, after pressing the send button browser shows only a blank page with no message.
Other think I would like you to help is the email address. If I am writing not correct email address, it sends the form anyway.
Thanks in advance.
<?php
if ($_SERVER['REQUEST_METHOD']=="POST"){
$to="mymail#example.com";
$subject="You have got an email from TGS";
$from = stripslashes($_POST['fname'])."<".stripslashes($_POST['email']).">";
if(empty($_POST['fname']) || empty($_POST['email']) || empty($_POST['message']))
{
$errors .= "\n Error: all fields are required";
}
$fname = $_POST['fname'];
$lname = $_POST['lname'];
$email = $_POST['email'];
$company = $_POST['company'];
$phone = $_POST['phone'];
$message = $_POST['message'];
if (!eregi( "^[_a-z0-9-]+(\.[_a-z0-9-]+)*#[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email))
{
$errors .= "\n Error: Invalid email address";
}
$mime_boundary="==Multipart_Boundary_x".md5(mt_rand())."x";
$tmp_name = $_FILES['filename']['tmp_name'];
$type = $_FILES['filename']['type'];
$file_name = $_FILES['filename']['name'];
$size = $_FILES['filename']['size'];
$message = "\n\n First Name: $fname \n\n Last Name: $lname \n\n Company: $company \n\n Phone: $phone \n\n Email: $email \n\nMessage: \n\n $message \n\nHere is your file: $file_name";
if (file_exists($tmp_name)){
if(is_uploaded_file($tmp_name)){
$file = fopen($tmp_name,'rb');
$data = fread($file,filesize($tmp_name));
fclose($file);
$data = chunk_split(base64_encode($data));
}
$headers = "From: $from\r\n" .
"MIME-Version: 1.0\r\n" .
"Content-Type: multipart/mixed;\r\n" .
" boundary=\"{$mime_boundary}\"";
$message = "This is a multi-part message in MIME format.\n\n" .
"--{$mime_boundary}\n" .
"Content-Type: text/plain; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
$message . "\n\n";
$message .= "--{$mime_boundary}\n" .
"Content-Type: {$type};\n" .
" name=\"{$file_name}\"\n" .
"Content-Transfer-Encoding: base64\n\n" .
$data . "\n\n" .
"--{$mime_boundary}--\n";
if (#mail($to, $subject, $message, $headers))
{
echo '<div><center><h1>Mail Sent successfully !!</h1></center></div>';
}else
{
echo '<div><center><h1>Error !! Unable to send Mail..</h1></center></div>';
}
}
}
?>