How to check extension of choosing picture in SP 2013 Picture Library

I need to check pictures extension when user has chosen picture and try to upload it into picture library.

I've found the way to edit master page with js script, but i haven't to edit master page. Then i try to use event receiver Adding, but it can't get the name of file or file path. I used:

var file = properties.ListItem.File.Name;//properties.ListItem - returns null

AfterProperties also returns null.

The another way i see is to edit Adding Picture form with js:

I think is's simplest way but i can't found information about it.

Problem: how to set js script to the form (see picture) or how to do such actions with another way