function Base64DecodeFile(ABase64Str: String; AFileName: String): Boolean;
var
  InSize, OutSize: Cardinal;
  PIn, POut: Pointer;
begin
  result:= False;
  // get size of source
  InSize := Length(ABase64Str);
  // calculate size for destination
  PIn := @ABase64Str[1];
  OutSize := CalcDecodedSize(PIn, InSize);
  // prepare string length to fit result data
  with TMemoryStream.Create() do
  try
    SetSize(OutSize);
    POut := Memory;
    // decode !
    if Base64Decode(PIn, InSize, POut) then
    begin
      SaveToFile(AFileName);
      result:= True;
    end;
  finally
    Free;
  end;
end;