Upload Operations: Validate and process files uploaded via Web forms

This class can be used to validate and process files uploaded via Web forms.

It can check if a file with a given form field name was uploaded and if its size it its within the specified file size limit.

It may also check if the file MIME type is one of those listed in the allowed file types class variable.

If all validation checks are satisfied, the class can move the file to a given directory for uploaded files.