We are currently investigating this in Windows only and will have an update out for iExplorer 4 for Windows very soon that restores the ability to add files to a folder on your device in the iExplorer window. In the meantime, it should work if you mount the folder into Windows Explorer as a disk. To do that, just right-clik on it in iExplorer and choose to Mount ‘folder name’ as disk. You may receive an error when drop/dropping files to the folder when mounted, but the files should still copy successfully despite the error. If you try to drag a file a 2nd time, you should get an error about the file already existing and if you navigate away from the folder and go back to it, then you should see the new file(s) that you added.
We should have the fix and update released very soon and bring iExplorer up to version 184.108.40.206. iExplorer updates automatically if you close reopen, or if there is an update available, you’ll see an Update button in the upper right corner, to the left of the Search field.
We appreciate everyone’s patience, and we apologize for any inconvenience this may cause any of you.