Fixes #8219. Introduces the mimeType option to override content-type header in conversion (and in native xhr when possible). Adds companion overrideMimeType method on jqXHR object (it simply sets the option). Unit test added.

src/ajax.js

@@ -426,6 +426,14 @@ jQuery.extend({
 					return match || null;
 				},
 
+				// Overrides response content-type header
+				overrideMimeType: function( type ) {
+					if ( state === 0 ) {
+						s.mimeType = type;
+					}
+					return this;
+				},
+
 				// Cancel the request
 				abort: function( statusText ) {
 					statusText = statusText || "abort";
@@ -827,7 +835,7 @@ function ajaxHandleResponses( s, jqXHR, responses ) {
 	while( dataTypes[ 0 ] === "*" ) {
 		dataTypes.shift();
 		if ( ct === undefined ) {
-			ct = jqXHR.getResponseHeader( "content-type" );
+			ct = s.mimeType || jqXHR.getResponseHeader( "content-type" );
 		}
 	}
 

src/ajax/xhr.js

@@ -87,6 +87,11 @@ if ( jQuery.support.ajax ) {
 						}
 					}
 
+					// Override mime type if needed
+					if ( s.mimeType && xhr.overrideMimeType ) {
+						xhr.overrideMimeType( s.mimeType );
+					}
+
 					// Requested-With header
 					// Not set for crossDomain requests with no content
 					// (see why at http://trac.dojotoolkit.org/ticket/9486)

test/unit/ajax.js

@@ -2169,6 +2169,34 @@ test("jQuery.ajax - transitive conversions", function() {
 
 });
 
+test("jQuery.ajax - overrideMimeType", function() {
+
+	expect( 2 );
+
+	stop();
+
+	jQuery.when(
+
+		jQuery.ajax( url("data/json.php") , {
+			beforeSend: function( xhr ) {
+				xhr.overrideMimeType( "application/json" );
+			},
+			success: function( json ) {
+				ok( json.data , "Mimetype overriden using beforeSend" );
+			}
+		}),
+
+		jQuery.ajax( url("data/json.php") , {
+			mimeType: "application/json",
+			success: function( json ) {
+				ok( json.data , "Mimetype overriden using mimeType option" );
+			}
+		})
+
+	).then( start , start );
+
+});
+
 test("jQuery.ajax - abort in prefilter", function() {
 
 	expect( 1 );