Simple fix for the CfP proposal progress issue

app/eventyay/cfp/flow.py

@@ -13,6 +13,7 @@
 from django.db.models import Q
 from django.forms import ValidationError
 from django.http import HttpResponseNotAllowed
+from django.utils.datastructures import MultiValueDict
 from django.shortcuts import redirect
 from django.urls import reverse
 from django.utils.functional import Promise, cached_property
@@ -219,14 +220,40 @@ def get_form_initial(self):
         return copy.deepcopy({**initial_data, **previous_data})
 
     def get_form(self, from_storage=False):
-        if self.request.method == 'GET' or from_storage:
+        # Cache form initial data to avoid repeated work
+        form_initial = self.get_form_initial()
+
+        if self.request.method == 'GET':
+            # For GET requests, use unbound forms with initial data
+            # This correctly displays saved values from the session
             return self.form_class(
-                data=self.get_form_initial() if from_storage else None,
-                initial=self.get_form_initial(),
+                data=None,
+                initial=form_initial,
+                files=None,
+                **self.get_form_kwargs(),
+            )
+        if from_storage:
+            # For validation checks, create a bound form with session data
+            return self.form_class(
+                data=form_initial,
+                initial=form_initial,
                 files=self.get_files(),
                 **self.get_form_kwargs(),
             )
-        return self.form_class(data=self.request.POST, files=self.request.FILES, **self.get_form_kwargs())
+        # For POST requests, merge new uploads with existing session files
+        # This allows users to navigate back without losing previously uploaded files
+        session_files = self.get_files() or {}
+
+        # Preserve MultiValueDict semantics for proper multi-file field support
+        files = MultiValueDict()
+        # Add session files first
+        for field, file_obj in session_files.items():
+            files[field] = file_obj
+        # New uploads take precedence over session files
+        for field, file_list in self.request.FILES.lists():
+            files.setlist(field, file_list)
+
+        return self.form_class(data=self.request.POST, files=files, **self.get_form_kwargs())
 
     def is_completed(self, request):
         self.request = request
@@ -237,11 +264,39 @@ def get_context_data(self, **kwargs):
         result['form'] = self.get_form()
         previous_data = self.cfp_session.get('data')
         result['submission_title'] = previous_data.get('info', {}).get('title')
+        # Add information about uploaded files for display in templates
+        saved_files = self.cfp_session.get('files', {}).get(self.identifier, {})
+        result['uploaded_files'] = {
+            field: file_dict.get('name') for field, file_dict in saved_files.items()
+        } if saved_files else {}
         return result
 
     def post(self, request):
         self.request = request
         form = self.get_form()
+        action = request.POST.get('action', 'submit')
+
+        # For "back" action, save data even if form is invalid
+        if action == 'back':
+            if form.is_valid():
+                self.set_data(form.cleaned_data)
+            else:
+                # Save raw POST data for invalid forms (user might be going back to fix something)
+                # Filter out non-field data and serialize appropriately
+                partial_data = {
+                    field_name: request.POST.get(field_name)
+                    for field_name in form.fields.keys()
+                    if field_name in request.POST
+                }
+                if partial_data:
+                    self.set_data(partial_data)
+            # Always save files if present
+            if form.files:
+                self.set_files(form.files)
+            prev_url = self.get_prev_url(request)
+            return redirect(prev_url) if prev_url else redirect(request.path)
+
+        # For "submit" and "draft" actions, validate as before
         if not form.is_valid():
             error_message = '\n\n'.join(
                 (f'{form.fields[key].label}: ' if key != '__all__' else '') + ' '.join(values)

app/eventyay/cfp/templates/cfp/event/submission_base.html

@@ -49,6 +49,16 @@ <h2>{% block submission_step_title %}{{ title|default:'' }}{% endblock submissio
             </div>
             {% block submission_step_text %}<p>{{ text|rich_text }}</p>{% endblock %}
             {% block inner %}
+                {% if uploaded_files %}
+                    <div class="alert alert-info mb-3">
+                        <i class="fa fa-info-circle"></i> {% translate "Previously uploaded files:" %}
+                        <ul class="mb-0 mt-1">
+                            {% for field_name, filename in uploaded_files.items %}
+                                <li><strong>{{ filename }}</strong> {% translate "(will be preserved if you don't upload a new file)" %}</li>
+                            {% endfor %}
+                        </ul>
+                    </div>
+                {% endif %}
                 {{ form }}
             {% endblock inner %}
             {% block buttons %}
@@ -76,9 +86,9 @@ <h2>{% block submission_step_title %}{{ title|default:'' }}{% endblock submissio
                     </div>
                     <div class="col-md-3">
                         {% if prev_url %}
-                            <a href="{{ prev_url }}" class="btn btn-block btn-info btn-lg">
+                            <button type="submit" class="btn btn-block btn-info btn-lg" name="action" value="back">
                                 « {{ phrases.base.back_button }}
-                            </a>
+                            </button>
                         {% endif %}
                     </div>
                 </div>

app/eventyay/cfp/views/wizard.py

@@ -71,6 +71,10 @@ def dispatch(self, request, *args, **kwargs):
                     or step.identifier == 'user'
                 ],
             )
+        if request.method == 'POST' and request.POST.get('action') == 'back':
+            # When clicking Back, the step's POST handler has already saved the data
+            # Now redirect to the previous step
+            return result
         if request.method == 'GET' or (step.get_next_applicable(request) or not step.is_completed(request)):
             if result and (csp_change := step.get_csp_update(request)):
                 result._csp_update = csp_change