Fix "ALT" being included in the post content of posts with images w/ alt text

[knuxify]

example 1 (embed): https://www.tumblr.com/punkitt-is-here/722387663671443456/my-family-got-two-koi-fish-and-i-got-to-name-them?source=share

example 2 (render): https://www.tumblr.com/nyancrimew/722661173729918976