<div class="form-group">
  <textarea id="solution_content" type="text" name="solution[content]" style="display: none"></textarea>
  <input id="upload-input" type="file" class="upload submission-control" size="5120" accept=<%= '.' + @exercise.language.extension %> />
</div>

<div class="form-group">
  <textarea id="solution_content" type="text" name="solution[content]" style="display: none"></textarea>
  <input id="upload-input" type="file" class="upload submission-control" size="5120" accept=".<%= @exercise.language.extension %>" />
</div>

Add missing quotemarks and remove concatenation
Add missing quotemarks and remove concatenation
