Liferay: Internet Explorer (IE) and JSON Mime Type
09 April 2016
The fix is to disable the JSONContentTypeFilter.
... com.liferay.portal.servlet.filters.jsoncontenttype.JSONContentTypeFilter=false ...
The issue is discussed in LPS-33187. IE 9 had no issue accepting "application/json" so it seems it is there to maintain compatibility with IE8 and earlier browsers?