var 
  vergl: array[0..11] of byte = 
  ($FF, $D8, $FF, $E0, 0, $10, $4A, $46, $49, $46, 0, 1); 
 
function isJPEG(Datei: string): boolean; 
var 
  ms: TMemorystream; 
begin 
  result := false; 
  if not fileexists(datei) then exit; 
  ms := Tmemorystream.create; 
  try 
    ms.loadfromfile(datei); 
    if ms.size > 11 then 
      result := comparemem(ms.memory, @vergl, 12); 
  finally 
    ms.free; 
  end; 
end; 
 
procedure TForm1.Button7Click(Sender: TObject); 
begin 
  if isJPEG('c:\test.jpg') 
    then showmessage('Ja') 
  else showmessage('Nein') 
end;