OK then, I think it’s MIME type related after all. It reminds me of this bug from last year. It’s fixed in libsoup 3.4; your libsoup is probably too old. Try updating that if possible.
Your WebKitGTK is new enough to have the fix for this issue (although you’re several months behind on security updates, which is inadvisable). And although the comment there also implies that you need an updated version of Epiphany as well, the change that landed there got reverted, so I don’t think Epiphany version matters. (Still, your Epiphany 42 is pretty old and using something newer would be advisable.)