How to accept both multipart and application/x-www-form-urlencoded?

I built a servlet that allows uploading a file or alternatively uploading a list as input. So far I've handled it by setting the servlet to accept multipart/form-data so even if there is no file, I read the list as a part.

I'm now trying to call this servlet to upload a list through a JQuery AJAX method instead of through a form. If I try to upload a list normally through the method, I get:

org.apache.tomcat.util.http.fileupload.FileUploadBase$InvalidContentTypeException: the request doesn't contain a multipart/form-data or multipart/mixed stream, content type header is application/x-www-form-urlencoded; charset=UTF-8

If I set the contentType as multipart/form-data like so:

$.ajax({        url: someUrl,     type: 'POST',     contentType: 'multipart/form-data',     data: {list: inputList}});

I get this error instead:

org.apache.tomcat.util.http.fileupload.FileUploadException: the request was rejected because no multipart boundary was found

My question is if there is some way to configure the servlet to accept both content types or alternatively is there some way to write the ajax data to upload multipart/form-data?

I know it's simple to upload multipart/form-data using the FormData API, but I need to support IE9 so this is not an option.