File Upload System: Validate and process file uploads

This class can be used to validate and process file uploads.

It can check whether the uploaded file has one of several allowed file name extensions, whether the file size does not exceed a given limit, and check whether a file with the same name was not previously uploaded.

A valid file is copied to a given destination directory. If the file is an image, it creates copy of the image in JPEG format that does not exceed a given width or height, if necessary scaling it and preserving the original proportions.