multipart/form-data combined with JSON post fields - php

Question

I'm trying to send a multipart/form-data form to a REST API. It should be possible to send files, but the other POST fields should be delivered as JSON.
That's how it should look like:
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Content-Disposition: form-data; name=entity
Content-Type: application/json
{
"vacancyId": "xxx",
"locationId": xxx,
"salutationId": xxx,
"firstname": "xxx",
"lastname": "xxx",
"xingProfile": "xxx",
"mobileNumber": "xxx",
"emailAddress": "xxx",
"comment": "xxx",
"mediaOptionId": xxx
}
---------------------------acebdf13572468
Content-Disposition: form-data; name="file"; filename="Archive.zip"
Content-Type: application/x-zip-compressed
<#INCLUDE *xxx\Archive.zip*#>
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Content-Disposition: form-data; name="file"; filename="CV.pdf"
Content-Type: application/octet-stream
<#INCLUDE *xxx\CV.pdf*
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I'm trying to solve this with PHP curl.
This is the current code for it:
$curl = curl_init();
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-type: ' . $contentType));
curl_setopt($curl, CURLOPT_POST, 1);
// ...
$data = json_decode($data, true); // data comes as JSON string
$tmpData = $data;
$tmpData['file'] = null; // remove files
$tmpData = (object) array_filter((array) $tmpData); // filter empty values
$json = json_encode($tmpData); // generate JSON entity
// generate data for posting
$postData = array();
$postData['entity'] = 'Content-Type: application/json\r\n\r\n' . $json;
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type' => 'multipart/form-data'));
// go through uploaded files
for ($i = 0; $i < count($data['file']); $i++) {
$file = $data['file'][$i];
$uploadedFile = curl_file_create($file['tmp_name'], $file['type'], $file['name']);
$postData['file'] = $uploadedFile;
}
curl_setopt($curl, CURLOPT_POSTFIELDS, $postData);
The request looks ok for me, but for some reason the API is not able to parse the JSON. I'm pretty sure it's because of Content-Type: application/json, which doesn't work like this. But maybe it's another problem. Any ideas?
This is the request, which the API gets:
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Content-Disposition: form-data; name="entity"
Content-Type: application/json\r\n\r\n{"vacancyId":xxx,"locationId":xxx,"salutationId":xxx,"firstname":"xxx","lastname":"xxx","emailAddress":"xxx","mobileNumber":"xxxx","comment":"xxx","mediaOptionId":xxxx,"xingProfile":"xxx"}
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PS: Files can be ignored for now. It seems to be a problem with the JSON part.