920328a872
if write_boot_content is given somethign that starts with #!,
then there isn't a reason to invoke 'file' to tell us that it
starts with shebang.
This way, we only run file in 2 cases:
a.) binary content (don't really know if that is supported or not)
b.) magic "user meant to run this with /bin/bash but couldn't be bothered to
type that"