type
  TByteArray = array of byte;

function FileReadAllBytes(fileName: string): TByteArray;
const
  readBufSize = 1023;
var
  res: TByteArray;
  myFile: File of byte;
  readBuf: array [0..readBufSize] of byte;
  i, j: integer;
  readInFact: integer;
begin
  AssignFile(myFile, fileName);
  Reset(myFile);
  SetLength(res, FileSize(myFile));
  i := 0;
  repeat
    blockread(myFile, readBuf, readBufSize+1, readInFact);
    for j := 0 to readInFact - 1 do
    begin
      res[i] := readBuf[j];
      i := i + 1;
    end;
  until i >= FileSize(myFile);
  Result := res;
end;

var
  test3: TByteArray;
....
test3 := FileReadAllBytes('bigfile.txt');
IdTCPClient1.WriteBuffer(Pointer(test3)^, Length(test3));