A typo snuck in #11492, which should've read `not type(article.mimetype) == "string" or type(article.mimetype) == "string" and not article.mimetype:find("^text/html")`. But in most cases the behavior would've been identically broken because of the same underlying issue: Wallabag mimetype is actually HTTP content-type.
Fixes#11528.
Also introduces a new setting associated with the behavior in case people have different preferences.
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