I resolved this by using the DeviceInfo FileExtension tags and also adding an ExtensionName for the TTX file output
<Render>
<Extension Name="XML" Type="Microsoft.ReportingServices.Rendering.DataRenderer.XmlDataReport,Microsoft.ReportingServices.DataRendering"/>
<Extension Name="NULL" Type="Microsoft.ReportingServices.Rendering.NullRenderer.NullReport,Microsoft.ReportingServices.NullRendering" Visible="false"/>
<Extension Name="TTX" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering">
<OverrideNames>
<Name Language="en-US">TTX for EDI</Name>
</OverrideNames>
<Configuration>
<DeviceInfo>
<FileExtension>TTX</FileExtension>
</DeviceInfo>
</Configuration>
</Extension>
<Extension Name="CSV" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering"/>
<Extension Name="TTX" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering"/>
<Extension Name="ATOM" Type="Microsoft.ReportingServices.Rendering.DataRenderer.AtomDataReport,Microsoft.ReportingServices.DataRendering" Visible="false"/>
<Extension Name="PDF" Type="Microsoft.ReportingServices.Rendering.ImageRenderer.PDFRenderer,Microsoft.ReportingServices.ImageRendering"/>
<Extension Name="RGDI" Type="Microsoft.ReportingServices.Rendering.ImageRenderer.RGDIRenderer,Microsoft.ReportingServices.ImageRendering" Visible="false"/>
<Extension Name="HTML4.0" Type="Microsoft.ReportingServices.Rendering.HtmlRenderer.Html40RenderingExtension,Microsoft.ReportingServices.HtmlRendering" Visible="false"/>
<Extension Name="MHTML" Type="Microsoft.ReportingServices.Rendering.HtmlRenderer.MHtmlRenderingExtension,Microsoft.ReportingServices.HtmlRendering"/>
<Extension Name="EXCEL" Type="Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer,Microsoft.ReportingServices.ExcelRendering"/>
<Extension Name="RPL" Type="Microsoft.ReportingServices.Rendering.RPLRendering.RPLRenderer,Microsoft.ReportingServices.RPLRendering" Visible="false" LogAllExecutionRequests="false"/>
<Extension Name="IMAGE" Type="Microsoft.ReportingServices.Rendering.ImageRenderer.ImageRenderer,Microsoft.ReportingServices.ImageRendering"/>
<Extension Name="WORD" Type="Microsoft.ReportingServices.Rendering.WordRenderer.WordDocumentRenderer,Microsoft.ReportingServices.WordRendering"/>
</Render>