GitLab "Upload a File" API returns "file is invalid" after POST

Question

I'm trying to upload an image from a WordPress backend to a GitLab project, such that I can use that image in an issue. The image comes from a ContactForm7 form and I store it in WordPress first.

GitLab has a specific API for uploading files use in issues, but no matter what I sent, it returns {"error":"file is invalid"}. Another Stackoverlow issue to a similar question exists, but the solution is incomplete and/or does not work. I don't want to commit an image, I'm fine with it being in this weird upload architecture of GitLab.

I tried using just the filename in the body, the internal path, the public URL to the image and a base64 encoded version. All tries lead to the same invalid file error. I can't find any documentation on how GitLab expects my image and what I need to do for GitLab to accept it. Any ideas?

This is my code:

function uploadToGitLab($contact_form) {
    $contact_form_id = $contact_form->id();
    $submission = WPCF7_Submission::get_instance();
    $data = $submission->get_posted_data();
    $files = $submission->uploaded_files();
    
    $markdownImage = "";
    if (!empty($files) && $contact_form_id == "721") {
        // move image from temp folder to upload folder
        $image_file = file_get_contents($files['anhang'][0]);
        $image_name = basename($files['anhang'][0]);
        $imageUpload = wp_upload_bits(basename($files['anhang'][0]), null, $image_file);    
        $image_filename = $imageUpload['url'];

        // Convert Image to Base64
        $image_type = pathinfo($imageUpload['file']);
        $image_stringdata = file_get_contents($imageUpload['file']);
        $image_base64 = base64_encode($image_stringdata);
        
        $uploadImageHeader = array(
            'Content-type' => 'multipart/form-data',
            // Set encoding to base64 if trying to send base64 image:
            'encoding' => 'base64'
        );
        $uploadImageBody = array(
            // Using the raw file uploaded in the form:
            //'file' => '@'. $files["anhang"][0]
            // Using just the filename:
            //'file' => '@'. basename($files["anhang"][0])
            // Using the image public URL:
            //'file' => '@'. $image_filename
            // Using base64 encoding of the image:
            'file' => '@' . $image_base64
        );
        $uploadArgs = array(
            'header' => $uploadImageHeader,
            'body' => $uploadImageBody
        );
        $imageResponse = wp_remote_post("https://example.com/api/v4/projects/MYPROJECTID/uploads?private_token=TOKEN", $uploadArgs);
        $markdownImage = $imageResponse["markdown"];
    }

As described above, no matter what I put into the 'file' => '@' . line, it seems to be invalid.

I tried:

• @image.png
• @/wordpress/uploads/path/to/image.png
• @https://example.com/image.png
• @base64encodedimage

But all return invalid. What is GitLab expecting?