+/** @brief Find the extension part of @p path
+ * @param path Path to parse
+ * @return Start of extension in @p path, or ""
+ *
+ * The return value may points into @p path and if so points at the "." at the
+ * start of the path. If the basename has no extension the result is "".
+ * Extensions are assumed to only contain the ASCII digits and letters.
+ *
+ * See also find_extension().
+ */