Hi nc,
According to the description, the following steps are for your reference:
1. Please follow Add Silverlight to a page using the Silverlight Web Part, and try to add the Silverlight Web Part into a SharePoint Online page again.
NOTE: After you have added the Silverlight application to the page, it is a good idea to test the different controls of the Silverlight application to make sure that they work.
2. If you have SharePoint on-premises environment, you can test the Silverlight Web Part in the on-premises environment to see if it works.
3. You can refer to Microsoft SharePoint Online for Enterprises Service Description to see blocked file types list.
For your reference:
SharePoint Online for Office 365 Developer Guide
SharePoint Online: An Overview for Developers
Thanks,
Cherry Wang