Trouble pulling OpportunityFileCollection associated with an Opportunity using OData service

Hello,

I'm trying to get the list of OpportunityFile associated with an Opportunity with no success.

I tried the following requests:

1.Using the link Url I'm getting a "Not Implemented" exception.

https://.bpmonline.com/0/ServiceModel/EntityDataService.svc/Opportunity…

<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
<script/>
<code>4
Not Implemented
Not Implemented

System.Data.Services.DataServiceException
at System.Data.Services.WebUtil.GetRequestEnumerator(IEnumerable enumerable)

 at System.Data.Services.DataService`1.SerializeResponseBody(RequestDescription description, IDataService dataService, IODataResponseMessage responseMessage)

 at System.Data.Services.DataService`1.HandleRequest()
Method 'SelectMany' not supported

System.NotSupportedException
at Terrasoft.Core.Entities.EntityQueryProvider.VisitMethodCall(MethodCallExpression node)

 at Terrasoft.Core.Entities.EntityQueryProvider.Build(Expression expression)

 at Terrasoft.Core.Entities.EntityQueryProvider.LoadEntityCollection(Expression expression)

 at Terrasoft.Core.Entities.EntityQueryProvider.ExecuteEnumerable(Type elementType, Expression expression)

 at Terrasoft.Core.Entities.EntityQuery`1.GetEnumerator()

 at System.Data.Services.WebUtil.GetRequestEnumerator(IEnumerable enumerable)

2. Using the $filter based on OpportunityId I'm receiving an exception saying the "OpportunityId not found".

https://.bpmonline.com/0/ServiceModel/EntityDataService.svc/Opportunity… eq guid''

<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
<script/>
<code>1
Collection item with name OpportunityId not found.
Collection item with name OpportunityId not found.

Terrasoft.Common.ItemNotFoundException
at Terrasoft.Core.Entities.EntitySchema.GetSchemaColumnByPath(String columnPath)

 at Terrasoft.Core.Entities.EntityQueryWhereBlockBuilder.CreateExpression(Expression node)

 at Terrasoft.Core.Entities.EntityQueryWhereBlockBuilder.CreateSimpleFilter(BinaryExpression binary)

 at Terrasoft.Core.Entities.EntityQueryWhereBlockBuilder.CreateFilter(Expression node)

 at Terrasoft.Core.Entities.EntityQueryWhereBlockBuilder.BuildBlock(MethodCallExpression node)

 at Terrasoft.Core.Entities.EntityQueryProvider.VisitMethodCall(MethodCallExpression node)

 at Terrasoft.Core.Entities.EntityQueryProvider.Build(Expression expression)

 at Terrasoft.Core.Entities.EntityQueryProvider.LoadEntityCollection(Expression expression)

 at Terrasoft.Core.Entities.EntityQueryProvider.ExecuteEnumerable(Type elementType, Expression expression)

 at Terrasoft.Core.Entities.EntityQuery`1.GetEnumerator()

 at System.Data.Services.WebUtil.GetRequestEnumerator(IEnumerable enumerable)

 at System.Data.Services.DataService`1.SerializeResponseBody(RequestDescription description, IDataService dataService, IODataResponseMessage responseMessage)

 at System.Data.Services.DataService`1.HandleRequest()

How can I get the list and avoid the exceptions? 

Thanks in advance.