If the file that you want to reference through an external entity is not an XML file, you must also specify the format of that file following the system identifier, as in this example for a PNG file.

如果您希望通过外部实体引用的文件不是XML文件，您必须在系统标识符之后指定该文件的格式，下面是一个使用PNG文件的例子。

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