Implemented option to append to an exisiting file

[Toflar]

This is a PR for #56.
@qzminski and @aschempp, please review.

[qzminski]

Looks good

[aschempp]

Can you explain why you changed the checkbox to a dropdown? What are the three different states? Create a new file, overwrite, or append? A blank option seems allowed, but I don't know what it would do?

[Toflar]

A blank option seems allowed, but I don't know what it would do?

It would neither override, nor append = create a new file every time you submit the notification :)

[aschempp]

That's not really clear with the selection, is it? Maybe we should add a blank option label?

[Toflar]

Well it wasn't there until now either. I could have added another checkbox to define if it should append but then you can check not check "override" and check "append" which totally makes no sense^^ But yeah, we could provide a blank option label.

[aschempp]

Either that or include a third option and make the field mandatory (which would be better with the class constants). The current implementation is taken from the Contao file upload field, but that does not support appending so we need to find something new.

[Toflar]

I have updated the PR :)

[aschempp]

👍