utils: In process_mime use env_vars.content_type
utils: In process_mime use env_vars.content_type

In process_mime(), use env_vars.content_type instead of
getenv("CONTENT_TYPE")

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@opentechlabs.co.uk>

ac000/receiptomatic