This is bit old post…just for info
var files = event.target.files
var filename = files[0].name
var extension = files[0].type
In the type you will find the extension
for eg: if it is jpeg image then,
extension = image/jpeg
or if pdf then,
extension = application/pdf
To obtain the exact value, perform extension.replace(/(.*)\//g, ”).
you will get the value.