Implemented MIMETypeForPathExtension for NSURL and NSString. refs #409

Refactored duplicated code for returning the MIME Type based on file path extension using
Core Services UTI.

Code/Support/NSURL+RestKit.m

@@ -21,6 +21,7 @@
 #import "NSURL+RestKit.h"
 #import "NSDictionary+RKAdditions.h"
 #import "RKFixCategoryBug.h"
+#import "NSString+RestKit.h"
 
 RK_FIX_CATEGORY_BUG(NSURL_RestKit)
 
@@ -30,4 +31,8 @@ RK_FIX_CATEGORY_BUG(NSURL_RestKit)
     return [NSDictionary dictionaryWithURLEncodedString:self.query];
 }
 
+- (NSString *)MIMETypeForPathExtension {
+    return [[self path] MIMETypeForPathExtension];
+}
+
 @end