对Mixcloud的POST请求

Question

I have a PHP script that POSTs to Mixcloud and receiving an error I have not been able to resolve.

This is using PHP 7.3.6 and curl as I've seen in other posts as example.

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"https://api.mixcloud.com/upload/?access_token=$mixcloud");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 60 * 10);
curl_setopt($ch, CURLOPT_NOPROGRESS, true);
$server_output = curl_exec($ch);
curl_close ($ch);
// load json response into array
$res = json_decode($server_output, true);
echo print_r( $post, true );
echo print_r($res, true);

The upload fails with the following post that was sent:

Array
(
    [mp3] => @/var/share_ro/fullshow/surface.mp3
    [name] => Upload title
    [description] => Upload description
    [picture] => @/var/share_ro/fullshow/avatar.png
    [tags-0-tag] => Tag1
    [tags-1-tag] => Tag2
    [tags-2-tag] => Tag3
    [tags-3-tag] => Tag4
    [tags-4-tag] => Tag5
)

and response:

Array
(
    [details] => Array
        (
            [mp3] => Array
                (
                    [0] => This field is required.
                )

        )

    [error] => Array
        (
            [message] =>
            [type] => PostValidationError
        )

)

However, the full path to the file does exist and I have tried with and without the leading @ on the file name:

$ ls -lah /var/share_ro/fullshow/surface.mp3
-rw-r--r-- 1 user mp3 79M Jun 11 16:08 /var/share_ro/fullshow/surface.mp3

Can someone see what it is I'm doing wrong?