Folder seen as file when connected as MTP
Because of the different way files and folders are handled on Windows when compared to other operating systems, it could be that the folder is corrupted and Windows is seeing it as a file (which it really is in the Android filesystem).  It may be possible to fix this problem by:


Creating a new folder,
Moving the contents of Backup into the new folder,
Removing Backup, and finally
Renaming the new folder to Backup


However, depending on what the underlying problem truly is, this may not fix it.