Looks up the MultipartResolver in Spring's root web application context.
Supports a "multipartResolverBeanName" filter init-param in web.xml;
the default bean name is "filterMultipartResolver". Looks up the MultipartResolver
on each request, to avoid initialization order issues (when using ContextLoaderServlet,
the root application context will get initialized after this filter).
MultipartResolver lookup is customizable: Override this filter's
lookupMultipartResolver method to use a custom MultipartResolver
instance, for example if not using a Spring web application context.
Note that the lookup method should not create a new MultipartResolver instance
for each call but rather return a reference to a pre-built instance.
Note: This filter is an alternative to using DispatcherServlet's
MultipartResolver support, for example for web applications with custom
web views that do not use Spring's web MVC. It should not be combined with
servlet-specific multipart resolution.