Refactor reject_illegal_file_attributes check

* Improve readability of method
* Add a new check for the author of a submission

spec/concerns/file_parameters_spec.rb

@@ -25,6 +25,8 @@ describe FileParameters do
 
       it 'new file' do
         submission = create(:submission, exercise: hello_world, id: 1337)
+        controller.instance_variable_set(:@current_user, submission.user)
+
         new_file = create(:file, context: submission)
         expect(file_accepted?(new_file)).to be true
       end
@@ -42,16 +44,27 @@ describe FileParameters do
         expect(file_accepted?(hidden_file)).to be false
       end
 
-      it 'read only file' do
+      it 'read-only file' do
         read_only_file = create(:file, context: hello_world, read_only: true)
         expect(file_accepted?(read_only_file)).to be false
       end
 
-      it 'non existent file' do
+      it 'non-existent file' do
         # Ensure to use an invalid id for the file.
         non_existent_file = build(:file, context: hello_world, id: -1)
         expect(file_accepted?(non_existent_file)).to be false
       end
+
+      it 'file of another submission' do
+        learner1 = create(:learner)
+        learner2 = create(:learner)
+        submission_learner1 = create(:submission, exercise: hello_world, user: learner1)
+        _submission_learner2 = create(:submission, exercise: hello_world, user: learner2)
+
+        controller.instance_variable_set(:@current_user, learner2)
+        other_submissions_file = create(:file, context: submission_learner1)
+        expect(file_accepted?(other_submissions_file)).to be false
+      end
     end
   end
 end