Uppercase extension ext.png not found

[zoaina]

Under linux server preview icon is not found if the extension is uppercase I suggest to fix this by putting this value to lower case

https://github.com/unclecheese/silverstripe-dropzone/blob/7058ff83420a779b7802c603873b6508ee66dfad/javascript/file_attachment_field.js#L392

to var ext = file.name.split('.').pop().toLowerCase();