The \middle command is not parsed in MediaWiki, though it can be quite useful in expressions like \left\{\frac ab\;\middle|\;a,b\in\mathbb Z\right\} (see rendering at https://commons.wikimedia.org/wiki/File:Middle_usage_example_LaTeX.svg). There appears to be a workaround at https://tex.stackexchange.com/questions/71284/a-command-to-replace-middle-when-its-missing, but that is quite unusual and not the intended way to do it.