Rated 5 out of 5 stars
I tried sending a large-ish (10MB) SVG file and it did its job as expected. Thunderbird 60.2.1 on Arch Linux. Megabird version 2.0.
Took around 20 seconds to upload the file on a normal broadband connection. It inserts a large HTML box with the link to the file.
I need to test if it works with plain-text messages. It would be nice to simply have a link included in the mail text. I need to test authentication with two factor auth (2FA) enabled on Mega too.
Will post updates after more testing.