But then again, you see files named .Jpg, and so on... Or even .jpG! And people who have (by default) extensions of for recognized file types in Windoze won't have a clue what the error is all about.

The smart way to do extension checking is to separate the file extension of the upload and normalize it to lower case, and compare that against your allowed values --- then you have only one possible match per extension, and need only one mention per extension type in your allowed arrays.