Should validate user's main entry

[iAmao]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

Currently I can type anything as my main entry file. It should not be so

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/84v837e9/).

N/A

What is the expected behavior?

I should only be allowed to use javascript files as my main entry in package.json.
If i type in something without a js extension then I should get an error
or better yet if it should append the .js for me.

Which versions of react-raise, and which browser / OS are affected by this issue? Did this work in previous versions of react-raise?

N/A

NB: You can drop a comment below for more clarification