Why don't you setup an email where they can send emails in specific format you define to them and just use Email Piping to pass email (txt) string(s) into your specific script on server.
PHP Email Pipe Tutorial, Read emails via php script | James Collings
It could be most user friendly solution for your users as they all have an email no need for free hosting to upload files, all text will be in email body that you can read and process with this technique.
This is how you usually open ticket in most ticket support systems and respond to same via an email.