The "Accepted file types" field was added to the Essay question in version 3.5. However, the XML import/export facility was not updated to support this new field. The steps to reproduce are as follows. 

1. Go to a course and navigate into the Course > Question bank page.
2. In the Question bank > Categories page, create a new category "Essay File Types".
3. Go to the Question bank > Questions page, select the "Essay File Types" category, and create a new Essay question.
4. Fill in the required fields, set the "Accepted file types" field to ".docx", and save the question.
5. Go to the Question bank > Export page, select the File format as "Moodle XML format", select the category "Essay File Types" in the "Export category" field, and click the "Export questions to file" button. The Essay question will be downloaded to a text file in XML format.
6. Open the file in a text editor and inspect the XML for the essay question. There is no element "<acceptedfiletypes>" or similar, so the export facility doesn't handle it.