Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
Hi BlueWolverine,
.mpp file doesn't support collaboration and there is no online previewer/editor for this type of file. This is the reason you see it force to download.
I believe what you want is to let others update the project details/status in a project. In this case, you will need to use Project Online and build up a Project Web App site. See this for more information, Project Online Admin Documentation - ProjectOnline | Microsoft Learn. After the site is created, everyone has necessary permission will be able to Connect to Project Online with the Project Online Desktop Client - ProjectOnline | Microsoft Learn via Project client or simply make changes directly in the site via browser.
One more thing, depending on how much function you need, you may also think about Project for the web if it matches you. Please note that Project for the web is web-based only. It doesn't support connection to Project client. See this if you would like to know more Project for the web and Project Online desktop client - Microsoft Support.
Regards,
Ryan | Community Moderator