SQL Server 2014 .bak file error - The media family on device 'D:\..' is incorrectly formed
There can be several causes for this error:


The backup is corrupted as it has not been transferred in binary mode If you are using FTP to transfer the backup, make sure the file is transferred in binary mode
The version of your backup is different than the SQL Server instance you are trying to restore it to. You can try to restore to a SQL server instance with the right version and transfer the data with SSIS
The backup is corrupt. In this case your options are limited