Cant Merge Pdf's with ConvertApi Rest api Flutter

I made the same requests for converting DOCX, excel.ppt to PDF they worked. But Merge PDF does not work. I select PDF's with file picker than I send request to convertApi . But ConvertApi returns my original pdf not merged pdf.

 var url = Uri.parse(
        'https://v2.convertapi.com/convert/pdf/to/merge?Secret=XXXX');

    int len = widget.docs.length;
    var request = MultipartRequest("POST", url);
    for (int i = 0; i < len; i++) {
      final element = widget.docs[i];
      request.files.add(
        await MultipartFile.fromPath(
          "Files",
          element.path,
          filename: element.name,
        ),
      );
    }

    var response = await Response.fromStream(await request.send());
    var decoded =
        base64.decode(await jsonDecode(response.body)["Files"][0]["FileData"]);

This is a CURL example, the Files parameter must have an index like this: files[0], files[1]. Please edit your Files parameter name and add bracket and index for every passed new file.

curl -F "Files[0]=@/path/to/file1.pdf" -F "Files[1]=@/path/to/file2.pdf" https://v2.convertapi.com/convert/pdf/to/merge?Secret=<YOUR SECRET HERE>