function StrToAnsiFileName(s: string): string;
var
i: integer;
begin
result := '';
for i := 1 to length(s) do
if s[i] in ['0'..'9', 'a'..'z', 'A'..'Z', '_', '.', ' '] then
result := result + s[i];
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage(StrToAnsiFileName('ÓáÇã adff 13.jpg'));
end;