function FileSizeToStr(const FileSize: Int64): string;
begin
 if FileSize = 1 then
   Result := '1 byte'
 else if FileSize < 1024 then
   Result := Format('%d bytes', [FileSize])
 else if FileSize < 1024 * 1024 then
   Result := Format('%lu KB', [FileSize / 1024])
 else if FileSize < 1024 * 1024 * 1024 then
   Result := Format(' MB', [FileSize / (1024 * 1024)])
 else
   Result := Format('%.3f GB', [FileSize / (1024 * 1024 * 1024)]);
end;

function FileSizeToString(FileSize: Integer): string;
const
 ONE_K = 1024;
 ONE_MB = 1024 * ONE_K;
 ONE_GB = 1024 * ONE_MB;
begin
 // Decide whether we should report filesize in GB, MB, KB or Bytes
 if FileSize >= ONE_GB then
   Result := Format('%.2f GB', [FileSize / ONE_GB])
 else if FileSize >= ONE_MB then
   Result := Format('%.2f MB', [FileSize / ONE_MB])
 else if FileSize >= ONE_K then
   Result := Format('%.2f KB', [FileSize / ONE_K])
 else
   Result := Format('%d Bytes', [FileSize]);
end;

function DoubleToStrEx(d: double; CountAfterDot: integer): string;
var a: integer;
begin
 Result := Double2Str(d);
 a := Pos('.', Result);
 if a >= 0 then
   Result := copy(Result, 1, a + CountAfterDot);
end;

function FileSizeToString(FileSize: Integer): string;
const
 ONE_K = 1024;
 ONE_MB = 1024 * ONE_K;
 ONE_GB = 1024 * ONE_MB;
begin
 if FileSize >= ONE_GB then
   Result := Format('%s GB', [DoubleToStrEx(FileSize / ONE_GB, 2)])
 else if FileSize >= ONE_MB then
   Result := Format('%s MB', [DoubleToStrEx(FileSize / ONE_MB, 2)])
 else if FileSize >= ONE_K then
   Result := Format('%s KB', [DoubleToStrEx(FileSize / ONE_K, 2)])
 else
   Result := Format('%d Bytes', [FileSize]);
end;