Files with HTML extensions are not showing up in the WYSIWYG Document Manager

Troubleshooting step 1: 

Confirm following the steps below if the HTML file extension is added to the list of allowed file types to your website:

• In the persona bar, Settings > Security > More tab > More Security Settings tab below More tab
• At the end of this page, you'll find all allowed extensions written in a textarea called "Allowable File Extensions", check if the HTML extension is there. 
• If it's not there, please make sure to add a comma and then the HTML extension at the end of the allowed file type list ( like this ",HTML")

Troubleshooting step 2: 

If you do have HTML added to the Allowable file extensions for the website you need to review the HTML Editor Manager configurations as below:

• Go to Host > Advanced Settings > HTML Editor Manager
• Click on 'everyone'
• Expand Document Manager Settings
• Check if any filters are applied (If there are filters applied you will need to add HTML)

Credits: Michael Ayoub