We have multiple .MPP (Microsoft Project Management) files that are not visible in search result in SharePoint Online.
The display form for the items are visible in search result but not the actual documents. Even after multiple calls with Microsoft support were I have shared my monitor and asked them why it doesn't work, they haven't been able to answer me.
I have found a couple of hints online, that .MPP files wasn't fully supported back in SP2010/SP2013 but nothing concrete on how they work today. I can understand if SharePoint Online doesn't support opening the files in browser but they should get indexed and shown in search result at least.