By default RichTextEditor automatically detect the MIME type of the files you upload, and rejects the file if the file-extension does not match the mime type ("The file is corrupt or has an incorrect extension"). You can turn it off by setting this property to "false".

public bool EnableMimetypeChecking { get; set; }

Public Property EnableMimetypeChecking As Boolean
	Get
	Set

public:
property bool EnableMimetypeChecking {
	bool get ();
	void set (bool value);
}