How to set file permissions after uploading a file via SFTP?
File permissions shall be set by specifying in File Transfer Task Editor > AdditionalOptions > permissions=[number]. 
The [number] is calculated as a sum of the selected enumeration items for SFTP permissions. See below an example that corresponds to a CHMOD 777 command for Unix-based servers:
In the above example "permissions=511" the [number] is specified to 511.
The number 511 is based on (sum of) the following enumeration and means that Others, Group and Owner can Read/Execute/Write:
  [Flags]
 
public enum SftpPermissions
 
{
    OthersExecute = 1,
     OthersWrite = 2,
    OthersRead = 4,    GroupExecute = 8,
    GroupWrite = 16,    GroupRead = 32,
    OwnerExecute = 64,    OwnerWrite = 128,
    OwnerRead = 256,    Sticky = 512,
    SetGid = 1024,
    SetUid = 2048,
  }