How to pass is_uploaded_file check in php?

I am uploading file to tmp folder from external file using php code(not from user post request), then fill $_FILES with information for that file and pass it to code that uses $_FILES variable. But inside that code, it uses move_uploaded_file, that has is_uploaded_file check. File uploaded manually by code(not by user form posted) fails this check because it was not uploaded using POST. Can I make it pass that check?

Code that fails:

//$url contains url of image from another site
function upload_from_url($url)
{
    global $modx;
    // we load temporary file to /tmp and then add it to $_FILES
    $img = '/tmp/'.uniqid("", true); // no extension here since we can't trust it
    $res = file_put_contents($img, file_get_contents($url)); // UPLOAD is made here, it works
    $mimeType = mime_content_type($img);
    $allowedMimeTypes = ["image/jpeg","image/gif","image/png"];
    if (!in_array($mimeType, $allowedMimeTypes))
    {
        logError("!!!ATTENTION!!! got file $img with forbidden mime type $mimeType");
        throw new RuntimeException("Forbidden file mime type!"); 
        die; // just to be sure; shouldn't get here
    }
    $_FILES['file']['name'] = uniqid("img", true)."."
        .(explode("/", $mimeType)[1]); // get jpg part of image/jpg
    $_FILES['file']['type'] = $mimeType;
    $_FILES['file']['tmp_name'] = $img;
    $_FILES['file']['error'] = 0; // = UPLOAD_ERR_OK
    $_FILES['file']['size'] = filesize($img);
    $response = $modx-> runProcessor('browser/file/upload', ['path' => 'assets/images']); // <= here it fails
}

Inside $modx-> runProcessor('browser/file/upload'... code, there is part that fails:

    if (!move_uploaded_file($file['tmp_name'],$newPath)) {
        $this->addError('path',$this->xpdo->lexicon('file_err_upload'));
        continue;
    }

It gets $file from $files from $_FILES variable, and I don't want to modify framework code to workaround it.