请问在react-native中 通过本地 url 获取的图片base64编码 怎么知道后缀名

Question

比如通过 getBase64ForTag 获取的base64编码如下
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不能通过直接给后缀名，因为可能有jpg,png或者gif,就算是给了后缀名.jpg还是会有部分图片显示不出来。
请问怎么获取它的后缀名？

另外我不想用react-native-image-crop-picker等插件。