MIME-info database contains multiple entries for *.png, namely image/png
and image/x-apple-ios-png. The later one doesn't have associated handler
program, but this one is selected by mimeopen tool.
Not sure how this tool should behave in case of multiple matches (IOW is
it a bug in File::MimeInfo perl module used by mimeopen). Instead of
switching to different tool, which probably will break other files
(check #423), add override for this particular file type.
