No icon / thumbnail for attached files when added via API

[apiening]

Infos:

• Used Zammad version: 3.6.x
• Installation method: Docker-Compose
• Operating system: Docker (Mac OS X)
• Browser + version: Firefox 83.0

Expected behavior:

When a file attachment is added to a ticket / article, it should be displayed with an icon for the given mime-type, or a thumbnail in case of an image file.

Actual behavior:

When a ticket / article has been created in the web interface or by incoming mail, the icon / thumbnail in front of the linked filename is shown correctly.
If the same entry gets created via API, an icon with a question mark (?) with a squared border is displayed instead of the actual thumbnail.

Steps to reproduce the behavior:

• Save the following PHP-script to a file (named addarticle.php for example):

<?php

//url
$baseurl = 'http://localhost';
$url = $baseurl . '/api/v1/ticket_articles?expand=true';

//Credentials
$client_id  = "username";
$client_pass = "password";

//ID of the ticket where this article should be added
$ticket_id = 40;

$create_article_data = [
    "ticket_id" => $ticket_id,
    "subject" => "some subject",
    "body" => "<h2>test</h2>huhuhuu<br>huhuhuu<br>huhuhuu<br><br>",
    "content_type" => "text/html",
    "type" => "note",
    "internal" => false,
    "attachments" => [
        [
            "filename" => "simpleimage.png",
            "data" => base64_encode(file_get_contents("simpleimage.png")),
            "mime-type" => "image/png",
        ]
    ]
];

$content = json_encode($create_article_data);

//HTTP options
$opts = array('http' =>
array(
    'method'    => 'POST',
    'header'    => array('Content-type: application/json', 'Authorization: Basic ' . base64_encode("$client_id:$client_pass")),
    'content' => $content
));

//Do request
$context = stream_context_create($opts);
$json = file_get_contents($url, false, $context);

$result = json_decode($json, true);
if (json_last_error() != JSON_ERROR_NONE) {
    return null;
}

print_r($result);

• Update the variables on top of the script to match your dev environment, especially $baseurl, $client_id, $client_pass and ticket_id.
• Run the script with php addarticle.php (or whatever your php file is named)