Fix ephy_file_browse_to() to ensure it selects the file

In cd0e4ef1 I "cleverly" reimplemented ephy_file_browse_to() to remove the need for the MIME type parameter, but it was too clever. The new implementation only opened the parent directory of the file without actually browsing to the file within that directory.

I don't want to revert, because I don't want ephy_file_launch_handler() to know about MIME types anymore. Instead, we can share code with ephy_file_open_uri_in_default_browser().