ImageUpload: Validate and process upload files

This class can be used to validate and process upload image and document files.

It takes the name of an file upload input field and validate its values to determine if it was uploaded a file of the accepted types and the file size does not exceed a given limit.

If the file is valid, it is copied to a given destination directory. The class creates that directory if it does not exist already.

If the file is an image, the class may also create a thumbnail image.