Eudyptula Challenge: Email provider sends base64 attachments and does not support POP3/IMAP
I signed up for the Eudyptula Challenge with an email address from Protonmail. Protonmail does not support POP3 or IMAP unless you have a paid account in which case they let you use their bridge application to set that up. Unfortunately, I do not pay for my account, so I'm stuck using their web interface which sends base64 attachments. Unlike disabling HTML composition, I have not been able to find a setting for this. I tried changing email used for the Eudyptula Challenge, but this resulted in me being told that if I did so my progression would not be reported to the challenge.
At this point I'm out of ideas apart from paying for a Protonmail account (and given what I learnt about their support for POP3/IMAP, or rather lack thereof, I'm not super interested in doing that). What might I do about this so that I can progress?