public class File
{
    public int ID { get; set; }
    public int Path { get; set; }
}

public class Lesson
{
    public int ID { get; set; }
    public virtual File File { get; set; }
}

public class Set
{
    public int ID { get; set; }
    public virtual List<File> Files { get; set; }
}

public class File
{
    public int ID { get; set; }
    public int Path { get; set; }
}

public class Lesson
{
    public int ID { get; set; }

    [ForeignKey("File")]
    [Index("IX_UniqueLessonFileId", IsUnique =true)]
    public int FileID { get; set; }

    public virtual File File { get; set; }
}

public class Set
{
    public int ID { get; set; }
    public virtual ICollection<SetFile> Files { get; set; }
}
public class SetFile
{
    public int ID { get; set; }

    [ForeignKey("Set")]
    public int SetId {get;set;}
    public virtual Set Set { get; set; }

    [ForeignKey("File")]
    [Index("IX_UniqueSetFileId", IsUnique =true)]
    public int FileId {get;set;}
    public virtual File File { get; set; }
}