Fix add_to_group and file type validation

lib/custom_wizard/actions.rb

@@ -138,7 +138,7 @@ class CustomWizard::Action
 
   def add_to_group
     groups = CustomWizard::Mapper.new(
-      inputs: action['inputs'],
+      inputs: action['group'],
       data: data,
       user: user,
       opts: {

lib/custom_wizard/builder.rb

@@ -305,7 +305,7 @@ class CustomWizard::Builder
       updater.fields[id] = standardise_boolean(value)
     end
     
-    if type === 'upload' && !validate_file_type(value, file_types)
+    if type === 'upload' && value.present? && !validate_file_type(value, file_types)
       updater.errors.add(id, I18n.t('wizard.field.invalid_file', label: label, types: file_types))
     end