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Of course you could add a server-side filter, but also having a client-side filter is a great UX for your users - no time wasted and no resources wasted to send a file to you and get back with an error.
You can do so by using the accept attribute, and specifying the MIME type of the files you accept.
image/* should catch all images.
<input type="file" name="myImage" accept="image/*" />
If you want to only allow some specific file types, list them:
<input type="file" name="myImage" accept="image/x-png,image/gif,image/jpeg" />
You can check the browser support for this attribute here: https://caniuse.com/#feat=input-file-accept