remove some code

app/views/exercises/_file_form.html.slim

@@ -1,5 +1,6 @@
 - id = f.object.id
-- file = form_files.file_id=id
+- file = object.file
+
 li.panel.panel-default
   .panel-heading role="tab" id="heading"
     a.file-heading data-toggle="collapse" data-parent="#files" href="#collapse#{id}"

app/views/exercises/_form.html.slim

@@ -2,7 +2,7 @@
 - file_types = FileType.where('file_extension IS NOT NULL').select(:file_extension, :id)
 
 = form_for(@exercise, data: {execution_environments: execution_environments, file_types: file_types}, multipart: true) do |f|
-  = render('shared/form_errors', object: @exercise, files: @files)
+  = render('shared/form_errors', object: @exercise)
   .form-group
     = f.label(:title)
     = f.text_field(:title, class: 'form-control', required: true)
@@ -35,7 +35,7 @@
   h2 = t('activerecord.attributes.exercise.files')
   ul#files.list-unstyled.panel-group
     = f.fields_for :files do |files_form|
-      = render('file_form', f: files_form, form_files: @files)
+      = render('file_form', f: files_form)
 
   a#add-file.btn.btn-default.btn-sm.pull-right href='#' = t('.add_file')
   ul#dummies.hidden = f.fields_for(:files, CodeOcean::File.new, child_index: 'index') do |files_form|