I just assumed, don't be offended. Response Content-type is always empty for me. I've never seen successful inlining. This is definitely not on my side.
Type of issue
IV not generated for target page
Reported
Mar 6, 2019
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Don't be ridiculous. This article is from today 10:23, I submitted my template yesterday at 18:11 and it works. At least for now: https://telegra.ph/file/55ec14ab5f6b96e7b7612.png
But as I said here: https://instantview.telegram.org/contest/nepszava.hu/template12/issue1/ there's a strange bug that gives this INLINE_MIME_TYPE_INVALID error, it seems random since the article in the linked issue gives it now too.
But your template fails on every page that has an image, not just some. Or is it working for you?