We have migrated our SharePoint 2010 farm to new SharePoint 2013 environment. Since we have a custom Master pages/Layout at the old environment, we used the old CSS/images to generate new Master pages/images/css to be used on new farm.
So far everything is working as old environment. But I just found if I change the site back to Oslo Master pages, users can drag and drop upload file to document library. However the "drag and drop area" is not existing with our custom Master.
What should I check to repair this feature? Any reference material is welcome. Thanks.