Edge option Open Office files in the browser not working

Hi @HIN 789,  

Welcome to Microsoft Q&A forum.  

Based on your concerns, to help clarify why you're seeing different behaviors, here’s a breakdown of what’s happening behind the scenes. Essentially, Edge’s internal settings are often overridden by the instructions it receives from the web server or the specific platform you're using. 

• Self-hosted servers: When you access a file from a general web server, the browser looks for specific headers in the response. If the server is configured to treat the file as an attachment, Edge will prioritize that instruction and download the file immediately, even if your browser setting is turned on. 
• About SharePoint Online: SharePoint is a bit more complex. It has its own built-in logic for handling documents to support features like co-authoring. The "Open this file?" prompt you're seeing is SharePoint’s way of handing the file over to its own web-based viewer or your local Office app, which can sometimes bypass the standard Edge viewer setting. 

For this to work seamlessly, the hosting environment needs to tell the browser that the file is safe to view "inline" rather than forcing a download. To help resolve this, I recommend the following: 

• Update Headers: Configure your self-hosted server to send Content-Disposition: inline and the correct Office MIME types. 
• SharePoint URL tip: Append ?web=1 to the end of your SharePoint file links to force the browser viewer. 
• Library settings: In SharePoint, set Opening Documents in the Browser to "Open in the browser" under Advanced Settings. 
• Extension check: Disable any third-party download managers that might be hijacking Office file requests. 

I hope this helps clarify the situation! If you have any questions, please feel free to reach out.