Attachments mime type is not actually stored in the database (NULL all the time)

Steps to reproduce:

• Create a new page, say TestPage.WebHome
• attach a new file to this page, favicon-ball.ico, for example.
• a query to the database with

  select XWA_FILENAME, XWA_MIMETYPE from xwikiattachment, xwikidoc where xwa_doc_id = xwd_id and XWD_fullname='TestPage.WebHome';

  shows

  +------------------+--------------------------+
  | XWA_FILENAME     | XWA_MIMETYPE             |
  +------------------+--------------------------+
  | favicon-ball.ico | image/vnd.microsoft.icon |
  +------------------+--------------------------+
  

• edit the document, type some text, save
• the same request to the database shows this:

  mysql> select XWA_FILENAME, XWA_MIMETYPE from xwikiattachment, xwikidoc where xwa_doc_id = xwd_id and XWD_fullname='TestPage.WebHome';
  +------------------+--------------+
  | XWA_FILENAME     | XWA_MIMETYPE |
  +------------------+--------------+
  | favicon-ball.ico | NULL         |
  +------------------+--------------+
  

So somehow the mimetype of the attachment gets stored but then gets deleted again from the database.

I am on MySQL database, default settings for attachments storage (database).