Check bin content file size before parsing request

This prevents MySQL being passed more data than it can handle and throwing an exception. The default limit is set to 1.5mb (although the actual value is in bytes) in the `"bin max-size"` option but might need to be tweaked for the production deploy.

Also this isn't handled in the front end and will currently fail silently. The request returns a `413` status code which we can check for and display an appropriate error message.