mpp files stored in SharePoint open read only in Project Client

Since I encountered this ever recurring question on our internal community sites and no clear explanation on it, here it is: Scenario : you have Microsoft Project Client installed and you create a new project. When saving, you decide to store the Project file (.mpp) in a SharePoint document list. You are able to successfully save the file. When you are trying to open the file directly from the SharePoint Document library, the project is opened always in read-only mode so you cannot save...(read more)