Fix: lookup JSDoc typings

[binury]

This is a quick fix to two functions which, since their detection failure returns false, have their return values annotated as {boolean|string}. In reality, the boolean returned is always false, and so it makes more sense to use a JSDoc type expression of false.

Sure. But why does this really matter?

With the return type as boolean i.e. {true|false}
a usage with a short circuit assignment to a {string} var will cause a false positive type warning:

const headers = {
  ⋮
  contentType: mime.lookup(path.extname(filePath)) || 'text/plain;charset=UTF-8',
}

as the language server infers from the {boolean|string} type annotation(s) that lookup will in some scenarios return true, which would make contentType no longer a {string}.

[binury]

any changes needed here to merge?