code: function image(input) {
          return new Promise(function (valid, invalid) {
              if (input.getAttribute('type') === 'file' && input.getAttribute('accept').indexOf('image') > -1 && _bn_getFile(input) !== false) {
                  BunnyFile.getSignature(_bn_getFile(input)).then(function (signature) {
                      if (BunnyFile.isJpeg(signature) || BunnyFile.isPng(signature)) {
                          valid();
                      } else {
                          invalid({ signature: signature });
                      }
                  }).catch(function (e) {
                      invalid(e);
                  });
              } else {
                  valid();
              }
          });
      }
docs: if file input has "accept" attribute and it contains "image", then check if uploaded file is a JPG or PNG