function TForm1.IsBMPInBLOB(BlobField: TBlobField): Boolean;
var
  BlobStream: TStream;
  SignatureBuffer: BITMAPFILEHEADER;
begin
  Result := False;
  if Assigned(BlobField) then
  begin
    BlobStream := BlobField.DataSet.CreateBlobStream(BlobField, bmRead);
    BlobStream.Seek( 0 ,  0 );
    if Assigned(BlobStream) then
    begin
      try
        BlobStream.Read(SignatureBuffer, SizeOf(SignatureBuffer));
        Result := (SignatureBuffer.bfType = $4D42) and
          (SignatureBuffer.bfSize = BlobStream.Size);
      finally
        BlobStream.Free;
      end;//try
    end;//if
  end;//if
end;