refactor and add using constants instead of hard coded strings ...

Providers/GraphQL/XFileGraphQLTypeHelper.cs

@@ -1,6 +1,7 @@
 using System;
 using xDataService.Configuration;
 using xDataService.GraphQL;
+using xFileService.Constants;
 using xFileService.Interfaces.Entities;
 using xFileService.Models.Entities;
 
@@ -15,12 +16,12 @@ namespace xFileService.Providers.GraphQL
 
         public override string GetInQueryCollectionName()
         {
-            return "files";
+            return XFileServiceConstants.XFileCollectionName;
         }
 
         public override string GetInQuerySingleName()
         {
-            return "file";
+            return XFileServiceConstants.XFileSingleName;
         }
     }
 }