Replace the content-type on ics files

[MelissaAutumn]

Should fix #773

I can't really test it in thunderbird, but the header shows up in mailpit.

I'm not sure if there's a better way to fix this header, the python api seems a little...light on implementation details outside of "Hey add an attachment to an email and send it!" 🤔

[devmount]

Wow, that's nice! Works on my end:

There is an option at the top right in Mailpit to save the email as raw. I just dragged that file into Thunderbird et voila!