Missing base64 attribute in request from "dotnet-svcutil" generated client

Bug: The generated client request is missing the `xsi:type="n1:base64" xmlns:n1="http://schemas.xmlsoap.org/soap/encoding/"` attribute on its `` element. The attribute specification is defined in the schema.

Schema: https://testing.maventa.com/apis/v1.1/wsdl
```
...

...
```

Generated request:
```
...

77u/PD94bWwgdmVyc2lvbj0iMS4wIi...ZvaWNlPg==
invoice1341515351353135.xml

...
```

Expected request:
```
...

77u/PD94bWwgdmVyc2lvbj0iMS4wIi...ZvaWNlPg==
invoice1341515351353135.xml

...
```

WSDL: https://testing.maventa.com/apis/v1.1/wsdl
Version: [Microsoft.Tools.ServiceModel.Svcutil, Version 2.0.0]

---
#### Document Details

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* ID: 3d0bd854-fec8-fc4a-760a-42dc22e07f99
* Version Independent ID: f04e9f8b-24d2-79fb-7521-6222c434e80f
* Content: [WCF svcutil tool overview - .NET Core](https://docs.microsoft.com/en-us/dotnet/core/additional-tools/dotnet-svcutil-guide?tabs=dotnetsvcutil2x)
* Content Source: [docs/core/additional-tools/dotnet-svcutil-guide.md](https://github.com/dotnet/docs/blob/master/docs/core/additional-tools/dotnet-svcutil-guide.md)
* Product: **dotnet-core**
* GitHub Login: @mlacouture
* Microsoft Alias: **mairaw**