The screenshot I've sent shows this in Chrome 57, where the left hand edge of the icon is clipped. All I had to do was follow the instructions in my original post.
The issue is even easier to produce in IE11 - I just went straight to your demo site (http://demos.telerik.com/aspnet-mvc/upload/validation) and uploaded a PNG file into the "PDF" upload control - the exclamation mark displayed has the left edge clipped off.
The same actions in Firefox 52.01 produce the same icon which is very slightly clipped to the right.
Also, the same client has raised another issue - with long file names. If you select a file with a very long file name and shrink the browser width, the file name will eventually lap over the bounds of the control. This is reproducible in Chrome, Firefox and IE.
The actual bug the client raised was during mobile browser testing - on a Galaxy S6. Please find attached the screenshot. Strangely, there seems to be different behaviour of the first and second upload controls in the screenshot.