function IsJpegFile(FileName: string): Boolean;

const

RightBuf : array[..] of Byte = ($FF,$D8,$FF,$D9);

var

Buf: array[..] of Byte;

begin

FillChar(Buf, , );

with TFileStream.Create(FileName, ) do begin

    Position := ;

    ReadBuffer(Buf[], );

    Position := Size-;

    ReadBuffer(Buf[], );

    Free;

end;

Result := CompareMem(@RightBuf[], @Buf[], );

end;

procedure TForm1.Button1Click(Sender: TObject);//测试

begin

if Self.OpenDialog1.Execute then

    if IsJpegFile(Self.OpenDialog1.FileName) then

      Showmessage('Is Jpg File');

end;