isBase64 function
Implementation
bool isBase64(String input) {
input = cleanBase64(input);
bool isBase64 = !input.contains(' ') &&
!input.contains('.') &&
((input.length % 4 == 0) &&
base64.decode(input).isNotEmpty &&
RegExp(r'^([A-Za-z0-9+/])*([A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$')
.hasMatch(input));
return isBase64;
}