Internet media types are also known as MIME types. The codeType property is read-only for any pre-existing object in an HTML document. For instances of objects that are created in script, the codeType property is read/write-once, because the codeType property is written when an object is created. After an instance of an object is created, the codeType property is read-only.
MIME Type Detection in Internet Explorer
Microsoft Developer Network: [Windows Internet Explorer API reference Article]