Use list of valid extensions to populate text in the upload template

Whether extensions are explicitly specified in settings, or else just
the default list of extensions is used, we should consider
importers.VALID_EXTENSIONS as canon and use that in user-facing templates

osgeo_importer/templates/osgeo_importer/new.html

@@ -39,6 +39,9 @@ <h2 class="page-title">{% trans "Add data" %}</h2>
             {% for error in form.file.errors %}
                 <i class="fa fa-warning">  {{ error|escape }}</i>
             {% endfor %}
+                <div class="col-md-6" style="padding-top: 5px; padding-left: 2px;">
+                    Valid file extensions: {{ VALID_EXTENSIONS }}
+                </div>
             </div>
 
         </div>

osgeo_importer/views.py

@@ -9,7 +9,7 @@
 from django.utils.text import Truncator
 from .forms import UploadFileForm
 from .models import UploadedData, UploadLayer, UploadFile, DEFAULT_LAYER_CONFIGURATION
-from .importers import OSGEO_IMPORTER
+from .importers import OSGEO_IMPORTER, VALID_EXTENSIONS
 from .inspectors import OSGEO_INSPECTOR
 from .utils import import_string, NoDataSourceFound
 from django.core.files.storage import FileSystemStorage
@@ -225,6 +225,9 @@ def form_valid(self, form):
 
     def render_to_response(self, context, **response_kwargs):
 
+        # grab list of valid importer extensions for use in templates
+        context["VALID_EXTENSIONS"] = ", ".join(VALID_EXTENSIONS)
+
         if self.json:
             context = {'errors': context['form'].errors}
             return self.render_to_json_response(context, **response_kwargs)