How to upload an attachment along with a Yammer message?

Any legacy method through attachment1 etc. fields of the /messages.json endpoint won't work anymore.

The new method is not so well documented: https://developer.yammer.com/docs/upload-files-into-yammer-groups

I'm giving here an example in PHP below, but you can do the same in any language.

You have to do it in two part

1. First upload the picture to https://filesng.yammer.com/v4/uploadSmallFile and get the picture's id.
2. Send your message as usual to https://www.yammer.com/api/v1/messages.json along with the freshly got picture id.

Note: I will use here the Guzzle library for the REST calls.

1. Send the picture to the Azure cloud

protected function yammerFileUpload(string $file, string $filename): int
{
    $multipart = [
        [
            'name'      => 'network_id',
            'contents'  => $this->networkId,
        ],
        [
            'name'      => 'group_id',
            'contents'  => $this->groupId,
        ],
        [
            'name'      => 'target_type',
            'contents'  => 'GROUP',
        ],
        [
            'name'      => 'filename',
            'contents'  => $filename,
        ],
        [
            'name'      => 'file',
            'contents'  => $file,
            'filename'  => $filename,
            'headers'   => ['Content-Type' => 'image/jpeg']
        ],
    ];

    $client = new Client();

    $options = [
        'headers'       => [
            'Accept'        => 'application/json',
            'Authorization' => "Bearer $this->yammerToken",
        ],
        'multipart'     => $multipart,
    ];
  
    $response = $client->request('POST', 'https://filesng.yammer.com/v4/uploadSmallFile', $options);

    return \json_decode((string)$response->getBody(), true)['id'];
}

Of course, you have to replace class variables with your owns. And the content type by your file's one.

2. Send your message

public function postMessage(string $message, string $file): array
{
    $fileId = $this->yammerFileUpload($file, 'my-file.jpg');

    $client = new Client();

    $options = [
        'headers'   => [
            'Accept'        => 'application/json',
            'Authorization' => "Bearer $this->token",
        ],
        'form_params' => [
            'body'               => $message,
            'group_id'           => $this->groupId,
            'attached_objects[]' => "uploaded_file:$fileId",
        ],
    ];

    $response = $client->request('POST', 'https://www.yammer.com/api/v1/messages.json', $options);

    return \json_decode((string)$response->getBody(), true);
}