Allow to cancel upload onFileUploadStart

It would be interesting to be able to filter files that we want uploaded or that we don't. For example, only allow files from fieldnames that are in the form we think files will be uploaded from.

This pull request offers that possibility, by returning {cancel: true} in the onFileUploadStart function.


I am open to any suggestions, I might be too careful in checking that the return value is really of the right type, and my variable name for that return value isn't that great.

If you agree on the design, I will edit the documentation to add that feature inside it.