Add functionality to remove a file

[sverhoeven]

Now the form looks like:

In the form you either must select a new file or keep the already present file. You can not delete an already present file.

Once you chosen a file in form and submit it you can only get rid of the file by reloading the page and starting over.
It would be nice if the files where removed when the node is removed.
Or even better if in the form there was a way to clear the chosen file.

[rvhonorato]

I think this was done with #29?

[sverhoeven]

In #29 you can remove all files. This issue is for removing a single file.

[sverhoeven]

@dmijatovic good first issue

[sverhoeven]

In file picker you can cancel the selection, which will clear the prev selection. Would be nice to add a note how to clear the selection.

[sverhoeven]

The emref module in distance constraints group has file uploads