Optional file inputs are not cleared if the user deselects the file

[ShaheedHaque]

Let say you have a with steps Form1 and Form2, where Form1 has a file input, which is optional. If the user takes the following actions:

• On Form1, enter filename x.txt.
• Progress to Form2.
• Go back to Form1, but this time the user does not select any file.
• Progress to Form2.

At this point, the expected behaviour is that:

1. self.storage.data[self.storage.step_files_key]['Form1'] should be empty.
2. self.storage.file_storage.delete() should have been called for the file for x.txt.

However, what actually happens is that the code in https://github.com/jazzband/django-formtools/blob/3b161670549503ae4ed81f9e0a43f2d12fc73e41/formtools/wizard/storage/base.py#L105 can only add selected files, and does not remove deselected files.

[ShaheedHaque]

Here is a prototypical fix. Change https://github.com/jazzband/django-formtools/blob/3b161670549503ae4ed81f9e0a43f2d12fc73e41/formtools/wizard/views.py#L297 to look something like this:

    #
    # The files storage is sticky w.r.t. optional files.
    #
    for file in list(self.storage.current_step_files or []):
        if file not in form.files:
            #
            # Remove the previously selected file.
            #
            file = self.storage.data[self.storage.step_files_key][self.steps.current].pop(file)
            self.storage.file_storage.delete(file['tmp_name'])
    self.storage.set_step_files(self.steps.current, self.process_step_files(form))

[ShaheedHaque]

Obsoleted in favour of #207.