Adding support for capital case file extension.

[eyal88]

Currently only lowercased file extensions works, even when trying to explicitly allow other cases via formats option like so:

L.Control.fileLayerLoad({
  formats: ['.gpx', '.GPX']
}

Loading filename.GPX for example returns a Unsupported file type (GPX) error.

With this fix, the following configuration supports both .gpx or .GPX:

L.Control.fileLayerLoad({
  formats: ['.gpx']
}