Multipart request processing in a Filter should be possible but is not defined

The section on file uploads specifies that the servlet container provides multipart/form-data processing if the Servlet is annotated with @MultipartConfig or the deployment descriptor contains a multipart-config element. Then the Servlet can use the HttpServletRequest getParts and getPart methods.

A Filter should also be able to call getParts but there is no way for a filter to be configured like a Servlet with multiple config markers.

An important use case to point out is providing CSRF protection (typically done with a Servlet) for multipart requests where the filter needs to access information about what parts are submitted in order to validate the request.