Question

I would like to determine real file extension for security reason.

How can I do that?

Answer 1

Supposing you really mean to get the true content type of a file (ie it’s MIME type) you should refer to this excellent answer.

You can get the true content type of a file in Java using the following code:

File file = new File("filename.asgdsag");
InputStream is = new BufferedInputStream(new FileInputStream(file));
String mimeType = URLConnection.guessContentTypeFromStream(is);