javascript: Not sending array with fetch api

I am trying to send array with fetch api. but it goes null.

javascript:

const url = "/TMF/DownloadFolderFilesAsZip";

        var data = {
            method: "POST",
            body: JSON.stringify({
                folderFiles: foldersFiles
            }),
            headers: new Headers({
                'content-type': 'application/json'
            })
        }

        fetch(url, data)
            .then(resp => resp.blob())
            .then(blob => {
                const url = window.URL.createObjectURL(blob);
                const a = document.createElement('a');
                a.style.display = 'none';
                a.href = url;
                a.download = "document" + ".zip";
                document.body.appendChild(a);
                a.click();
                window.URL.revokeObjectURL(url);
            })
            .catch(() => alert(''));

data:

(2) [{…}, {…}]
0: {fileId: '', folderId: 'e48634fb-23eb-45fa-b974-c83a3032db63'}
1: {fileId: '', folderId: 'c3bdd287-4461-4b5e-95c9-4048e8f34e58'}
length:2
[[Prototype]]: Array(0)

controller:

[HttpPost]
        public async Task DownloadFolderFilesAsZip([FromBody] List<TmfZipFolderFiles> folderFiles)
        {
           ......
        }

c# class:

public class TmfZipFolderFiles
    {
        public Guid fileId { get; set; }
        public Guid folderId { get; set; }
        public Guid tenantId { get; set; }
        public Guid userId { get; set; }
    }

I can’t send the array with the above codes. null data goes to the controller. How can I send the array?