initBinder method is not being called for conversion from String to Enum

I'm testing out the @InitBinder annotation so I can have String objects converted into appropriate Enum objects during web requests.

I created the following simple Enum:

SampleEnum.java

public enum SampleEnum {
    ONE,
    TWO,
    THREE,
    FOUR,
    FIVE;
}

Then, I created an editor extending PropertyEditorSupport to be called from the @InitBinder code:

EnumPropertyEditor.java

@SuppressWarnings("rawtypes")
public class EnumPropertyEditor extends PropertyEditorSupport {
    private Class clazz;

    public EnumPropertyEditor(Class clazz) {
        this.clazz = clazz;
    }

    @Override
    public String getAsText() {
        return (getValue() == null ? "" : ((Enum) getValue()).name());
    }

    @SuppressWarnings("unchecked")
    @Override
    public void setAsText(String text) {
        Enum e = Enum.valueOf(clazz, text);
        setValue(e);
    }
}

Then, in my controller I added the @InitBinder and a simple request mapping:

Controller

@InitBinder
public void initBinder(WebDataBinder binder) {
    binder.registerCustomEditor(SampleEnum.class, new EnumPropertyEditor(SampleEnum.class));
}

@RequestMapping(method = POST, value = "/postSampleEnum")
@ResponseBody
public SampleEnum postSampleEnum(@RequestBody SampleEnum sampleEnum) {
    return sampleEnum;  
}

From my understanding, a request for this controller method should attempt to convert a string value into the SampleEnum object. However, no breakpoints are hit in either initBinder, request mapping method, nor any of the methods in the EnumPropertyEditor.

I'm testing with RESTClient in FireFox, and have tried sending in the request body "THREE", which I would expect to work. Instead, I get a 415 error regardless of the what's in the request body. (The server refused this request because the request entity is in a format not supported by the requested resource for the requested method ().)

If I change the request mapping to take in a string instead of a SampleEnum, the postSampleEnum gets called and doesn't use the custom editor (as expected).

Am I missing anything that allows the custom editor code to be called? What is the best way to continue debugging this?

First of all, I forgot to add the application/json content-type to the request header in RESTClient. >_<

However, I noticed that the code execution still doesn't go through the custom property editor. As GriffeyDog said, it looks like the code only executes if I switch to a RequestParam or ModelAttribute.