Beginning in mid-2015, Microsoft integrated Office Online file preview integration with Yammer. With the release of Office 365 Group-connected Yammer sites, Microsoft made the decision to start migrating content storage from Yammer-based file storage to Office 365 SharePoint online file storage.
Integration is enabled by default and requires no configuration by users. If, however, you previously had documents stored in Yammer storage and the migration has been completed for your tenant, your Yammer-stored files will have been marked read-only. In order to enable document collaboration on those documents, you will need to save the documents from the Yammer group and reupload them to allow them to be saved in the connected group's SharePoint site.