Motivation:

• You have a WordPress instance on Windows IIS.
• You upload a file. Its thumbnail is not shown in Media Library.
• You change the file permission. Its thumbnail now is shown correctly in Media Library.
• You upload another file and have to change the file permission manually again.
• How can we make WordPress automatically set the correct permission for new uploaded files?

Procedure:

1. Ensure that the the Identity of Application pool that the website is running under is ApplicationPoolIdentity.
2. Execute below commands as Administrator
   

icacls "C:\inetpub\wwwroot\domain.com" /grant "IUSR":(OI)(CI)F /T 
icacls "C:\inetpub\wwwroot\domain.com" /grant "IIS_IUSRS":(OI)(CI)F /T

3. Set up IIS.

• Open IIS Manager.
• Click on your website.
• Click Authentication.
• Click Anonymous Authentication (which should be the only one enabled).
• Click Edit.
• Select Application pool identity if it is not selected.
• Click OK.