improve security for file upload

[GeorgeGoodall]

Ticket: https://trello.com/c/v5JRhYVn/1177-ensure-safe-upload-of-files

This ticket improves the security checks for the file that is uploaded

it does this by adding checks for validating

• the extension
• the size
• the filename

It also ensures files are deleted after the have been processed

[cjohns-scottlogic]

Can we make this check cover both the is valid extension and is valid mime type ones? (although we still need to check there is exactly only one extension)