docx mimetype returning incorrect "application/octet-stream"

from jquery fileupload control uploading word document(.docx), it is returning mimetype as "application/octet-stream" instead of "application/vnd.openxmlformats-officedocument.wordprocessingml.document".

Issue is happening only for docx, but it is working fine for jpeg,pdf.

Note: Here uploadFileFormat is collection, it has all allowed fileformats.

Please help some one.

$('#fileupload').fileupload({
  dataType: 'json',
  type: "POST",
  url: getURL(),
  beforeSend: function(xhr) {
    var securityToken = $('[name=__RequestVerificationToken]').val();
    xhr.setRequestHeader('__RequestVerificationToken', securityToken);
  },
  maxNumberOfFiles: 2,
  getNumberOfFiles: function() {
    return 2
  },
  autoUpload: true,
  done: function(e, data) {
    successUpload(data);
    IsFileUpload = false;

  }
});

HttpPostedFileBase objHPF = Request.Files[file] as HttpPostedFileBase;
                    if (objHPF.ContentLength == 0)
                        continue;
                    if (!uploadFileFormat.Contains(objHPF.ContentType.Replace("application/", "").Replace("image/", "")))
                    {
                        return Content("{\"status\":\"fail\",\"message\":\"Incorrect file format\"}", "application/json");
                    }