Attachments sent with imap_mail_compose in PHP rename files with .dat - php

Question

I am creating a .eml file with the following code to create a "draft" email that can be open as draft in Outlook and ready to be sent.
I got it working 99%, but the email generated has the attachments with .dat extension.
For example, and excel file will be name myfile.xlsx.dat ; if I take the file and remove the .dat extension i can open it in Excel. Same thing for pdf attachment, gets renamed .dat, but remove the .dat and it opens as a valid PDF.
How can I get rid of of the .dat extension?
$part1["type"] = TYPEMULTIPART;
$part1["subtype"] = "mixed";
$fp = fopen($full_filename2, "r");
$contents = fread($fp, filesize($full_filename2));
fclose($fp);
$part2["type"] = TYPEAPPLICATION;
$part2["encoding"] = ENCBINARY;
$part2["subtype"] = "octet-stream";
$part2["description"] = basename($full_filename2);
$part2["contents.data"] = $contents;
$fp = fopen($full_filename3, "r");
$contents = fread($fp, filesize($full_filename3));
fclose($fp);
$part3["type"] = TYPEAPPLICATION;
$part3["encoding"] = ENCBINARY;
$part3["subtype"] = "octet-stream";
$part3["description"] = basename($full_filename3);
$part3["contents.data"] = $contents;
$fp = fopen($full_filename, "r");
$contents = fread($fp, filesize($full_filename));
fclose($fp);
$part5["type"] = TYPEAPPLICATION;
$part5["encoding"] = ENCBINARY;
$part5["subtype"] = "octet-stream";
$part5["description"] = basename($full_filename);
$part5["contents.data"] = $contents;
$part4["type"] = TYPETEXT;
$part4["subtype"] = "html";
$part4["description"] = "body";
$part4["contents.data"] = $email_body;
$body[1] = $part1;
$body[2] = $part2;
$body[3] = $part3;
$body[4] = $part4;
$body[5] = $part5;
$singlefile = uniqid(rand(), true) . '.eml';
$imapfile = 'C:\\wamp\\www\\billing\\emails\\'.$singlefile;
file_put_contents($imapfile , imap_mail_compose($envelope, $body));