In CorsFilter of trunk

starting with line 1030:
[[[
    /**
     * {@link Collection} of Simple HTTP request headers. Case in-sensitive.
     *
     * @see  <a href="http://www.w3.org/TR/cors/#terminology"
     *       >http://www.w3.org/TR/cors/#terminology</a>
     */
    public static final Collection<String> SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES =
            new HashSet<>(Arrays.asList("application/x-www-form-urlencoded",
                    "multipart/form-data", "text/plain"));
]]]

starting with line 641:
[[[
                    } else if ("POST".equals(method)) {
                        String contentType = request.getContentType();
                        if (contentType != null) {
                            contentType = contentType.toLowerCase().trim();
                            if (SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES
                                    .contains(contentType)) {
                                requestType = CORSRequestType.SIMPLE;
                            } else {
                                requestType = CORSRequestType.ACTUAL;
                            }
                        }
                    } else {
]]]

According to w3.org "Terminology" link above,
1) Javadoc for SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES is a bit off (copy-pasted from another field). It is actually a collection of mime-type values for Content-Type header so that this header is treated as a "simple header"

2) The check using SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES shall ignore any parameters that may be present in Content-Type header value.

E.g. "Content-Type: application/x-www-form-urlencoded; charset=UTF-8"  shall match successfully.