first try...

app/views/exercises/_file_form.html.slim

@@ -1,4 +1,5 @@
 - id = f.object.id
+- file = form_files.file_id=id
 li.panel.panel-default
   .panel-heading role="tab" id="heading"
     a.file-heading data-toggle="collapse" data-parent="#files" href="#collapse#{id}"
@@ -37,3 +38,4 @@ li.panel.panel-default
           = f.label(:role, t('activerecord.attributes.file.weight'))
           = f.number_field(:weight, class: 'form-control', min: 1, step: 'any')
       = render('code_field', attribute: :content, form: f, label: t('activerecord.attributes.file.content'))
+      .editor data-file-id=file.ancestor_id data-indent-size=file.file_type.indent_size data-mode=file.file_type.editor_mode data-read-only=file.read_only data-id=file.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)
+  = render('shared/form_errors', object: @exercise, files: @files)
   .form-group
     = f.label(:title)
     = f.text_field(:title, class: 'form-control', required: true)
@@ -35,7 +35,8 @@
   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)
+      = render('file_form', f: files_form, form_files: @files)
+
   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|
     = render('file_form', f: files_form)