How to restrict a file if file extension is changed and try to upload using plupload

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Javascript

Java

.NET

upload

I am using Puploader to upload file in a java project. I am restricting some of the filetypes like .exe from uploading. But when the extension is changed it gets uploaded. For example, when a user changed the extension of the test.exe file to test.txt and tried to upload, its get uploaded in the project. This cause serious issue in my project as the file is downloadable by other users.
So I need to restrict the files by checking the previous extensions or if possible by checking the file content. Is there any way for that?
If not possible by using Plupload, please suggest a way to do it in javascript.

What I have tried:

Now, I fetch the extension from the filename using substring in javascript. I need a way to find out the original filetype even if the extension has changed.