Workaround for paid users for as long as this is not implemented:
Download Protonmail-Bridge, set-up Thunderbird (other e-mail clients will probably work too, but I haven't tried it)
Find the e-mail you want to delete an attachment from, copy the e-mail to Thunderbird's local folders. Delete the attachment there. Delete the original e-mail on protonmail. At last, copy the e-mail from your local folders back to protonmail.
For some reason, deleting attachments of e-mails that are still on protonmail would delete the whole e-mail. That's why you need to copy it to your local folders.
One major downside of this is that thunderbird will leave you a message that an attachment was deleted from this e-mail. This message will be copied to protonmail, too. It's a bit ugly, but it works.
Sebastian
Workaround for paid users for as long as this is not implemented:
Download Protonmail-Bridge, set-up Thunderbird (other e-mail clients will probably work too, but I haven't tried it)
Find the e-mail you want to delete an attachment from, copy the e-mail to Thunderbird's local folders. Delete the attachment there. Delete the original e-mail on protonmail. At last, copy the e-mail from your local folders back to protonmail.
For some reason, deleting attachments of e-mails that are still on protonmail would delete the whole e-mail. That's why you need to copy it to your local folders.
One major downside of this is that thunderbird will leave you a message that an attachment was deleted from this e-mail. This message will be copied to protonmail, too. It's a bit ugly, but it works.