when we download the file its in .txt format. As per the hint the file heaer is corrupted so we need t correct it. Before correcting thre fle header i checked what type of file.txt was using zsteg to reveal what type of file it was, it revealed it was a PNG file with corrupted header. So opened the file in Ghex and corrected the file.txt header to PNG file signature and saved it as .png file.
After that viewed the newly created file.png and got the flag:
nactf{th3_turn1p5_ar3_tak17g_0v35_skf9}