Wikitext's wikilink syntax is overloaded.
[[File:Test.png]] will produce an image.
In order to link to the page File:Test.png,
[[:File:Test.png]] should be used.
However, if a link is prefixed with multiple colons, (i.e.
[[::File:test.png]]), every colon after the first is considered part of title. These are invalid links and will be rendered as plaintext.
Till recently (before https://gerrit.wikimedia.org/r/#/c/361597/ was deployed), there was some leniency around the number of prefix colons permissible in wikilinks, but this is no longer the case. See the discussion on wikitech-ambassadors.
This linter category will uncover wikilinks using multiple colons as a prefix so editors can inspect and fix them, as required.