Как определить графический формат файла (не используя расширение)?

You can determine it without extention. Below is a function that reads the file header and determines the type.

function PhysicalResolveFileType(AStream: TStream): Integer;
var
  p: PChar;
begin
  Result := 0;
  if not Assigned(AStream) then
    Exit;
  GetMem(p, 10);
  try
    AStream.Position := 0;
    AStream.Read(p[0], 10);
    {bitmap format}
    if (p[0] = #66) and (p[1] = #77) then
      Result := 1;
    {tiff format}
    if ((p[0] = #73) and (p[1] = #73) and (p[2] = #42) and (p[3] = #0)) or
      ((p[0] = #77) and (p[1] = #77) and (p[2] = #42) and (p[3] = #0)) then
      Result := 2;
    {jpg format}
    if (p[6] = #74) and (p[7] = #70) and (p[8] = #73) and (p[9] = #70) then
      Result := 3;
    {png format}
    if (p[0] = #137) and (p[1] = #80) and (p[2] = #78) and (p[3] = #71) and
      (p[4] = #13) and (p[5] = #10) and (p[6] = #26) and (p[7] = #10) then
      Result := 4;
    {dcx format}
    if (p[0] = #177) and (p[1] = #104) and (p[2] = #222) and (p[3] = #58) then
      Result := 5;
    {pcx format}
    if p[0] = #10 then
      Result := 6;
    {emf format}
    if (p[0] = #215) and (p[1] = #205) and (p[2] = #198) and (p[3] = #154) then
      Result := 7;
    {emf format}
    if (p[0] = #1) and (p[1] = #0) and (p[2] = #0) and (p[3] = #0) then
      Result := 7;
  finally
    Freemem(p);
  end;
end;

Взято с Delphi Knowledge Base: https://www.baltsoft.com/