Introduce strict server side validation of file types in the file whitelist

Based on a recent IT Healthcheck finding we would like Cerberus to implement strict server side validation of the file types being uploaded. It has been identified that simply changing the extension of the file type (e.g. .exe. to .exe.csv) allowed the file whitelist to be bypassed. The recommendation from the testers is that the server should validate both the magic number and mime type of the file being uploaded and reject the upload of any file where the content includes forbidden types regardless of file extension.