isValidFileExtension method
Returns true if string is a file
Implementation
bool isValidFileExtension([List<String>? allowed]) {
if (this == null) {
return false;
}
final extensions =
allowed ?? ['.jpg', '.jpeg', '.png', '.pdf', '.docx', '.txt', '.json'];
final ext = this!.toLowerCase().split('.').last;
return extensions.any((e) => '.$ext' == e.toLowerCase());
}