text: Title: MIME Type of the generated xlsx file
  Body: Title: Issue with MIME Type of the generated xlsx file
  
  Hello,
  
  I am using XlsxWriter to generate excel files (obviously), no problem on the generation part but when I check the MIME type of the generated file, it's not what it should be.
  
  I am using Python version 3.7 and XlsxWriter 1.1.5 and Excel version 16.23 (Excel for Mac).
  
  Here is the code to demonstrates the problem:
  
  ```python
  
  from xlsxwriter import Workbook
  
  wb = Workbook("created_with_xlsxwriter.xlsx")
  ws = wb.add_worksheet()
  ws.write(0, 0, "Hello")
  wb.close()
  
  ```
  
  And when I check the MIME type of the file with the following command:
  ```
  > file --mime-type -b created_with_xlsxwriter.xlsx
  application/zip
  ```
  
  If I do the same command on a file created with excel:
  ```
  > file --mime-type -b created_with_excel.xlsx
  application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
  ```
  
  The strangest thing is that if I open the generated file in Excel and do ```command + s``` on my Mac, and try the above command again, the MIME type become the good one. 
  Same happens if I open the file with a PHP xlsx library and save it again.