Where does uploaded file goes when a user upload a file?

I am building a simple web project that allows people to upload the image file (all image extensions are accepted) and encrypt it. The project I am building does not have a server and the encryption process is done in the browser. So the question is when a user uploads an image file, where does it go? Because I am concerned that people can modify the executable to image extension and upload it to my system.

Thank you very much for your time and have a nice day.