'appIcon' and 'contentImage' options could actually accept data:, http(s):, etc URIs. However, the fact that the current version checks explicitly for file:// URIs cripples that use case. This commit tries to treat the given string as a URI. However, if the URI does not have a scheme, then 'file://' is appended and the resulting string is re-interpreted as the final URI. Otherwise, we are set anyway.