Merge pull request #316 from guidomb/linux

Change NSString cast to avoid linux build failure.

Sources/MimeTypes.swift

@@ -136,7 +136,7 @@ extension NSString {
 
 extension String {
     public func mimeType() -> String {
-        return (self as NSString).mimeType()
+        return (NSString(string: self)).mimeType()
     }
 }