multipart/form-data with both files and text-based form data in App Inventor - app-inventor

Question

I am trying to build a small app that will take a multipart/form-data to send a form with two data items:
username - a text field
file - an image file
The app blocks are attached in the picture to this mail. I do not know how to send multipart/form-data with both files and text-based form data. Need your suggestions on this. Can you please help?
For those who are not acquainted with the Content-Type=multipart/form data, here is a one liner about it:
The content type "application/x-www-form-urlencoded" is inefficient
for sending large quantities of binary data or text containing
non-ASCII characters. The content type "multipart/form-data" should be
used for submitting forms that contain files, non-ASCII data, and
binary data.
More details can be found in:
https://www.w3.org/TR/html401/interact/forms.html#successful-controls
I have found ways to achieve it using get and put methods. I would want to achieve this using the Post method.
The php server code is:
<?php
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$user = $_POST["username"];
$uploadOk = 1;
$imageFileType = pathinfo($target_file,PATHINFO_EXTENSION);
if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
} else {
echo "Sorry, there was an error uploading your file.";
$uploadOk = 0;
}
if ($uploadOk == 0) {
echo "Upload failed with reason = $uploadFailureReason <br>";
}
else {
echo "Upload Sucessful <br>";
$pyscript = '/home/ubuntu/workdir/src/httppython.py';
$python = '/home/ubuntu/.virtualenvs/cv/bin/python';
$filePath = $target_file;
$cmd = "$python $pyscript --image $filePath --user $user";
//echo "command = $cmd ";
exec("$cmd", $output);
echo "<input type=button onClick=\"location.href='upload_new.html'\" value='New Upload'>"
}
?>