This happens because your browser is trying to open the package files it downloads (and, of course, browsers can't read package files so that they will generate an 'unknown format' error).
You can either ignore the error (your downloads will be unaffected) or open your browser's menu to find where this setting is so you can disable or change it. Note: all browsers work a little differently, so simply search for yours with something like "stop [name-of-browser] opening files automatically" (where the name of the browser will be something like Edge, Chrome, Firefox, Opera, Vivaldi, Safari, or Brave for example).
For example: to stop downloads being opened automatically in Chrome:
- Click the three (vertical) dots in the upper right corner of the Chrome browser window to access its menu
- Select Settings
- Expand the Advanced section
- Under the Downloads section, click the CLEAR button next to Open certain file types automatically after downloading.
