I have not been able to get this to work natively in C#, but I do have a work around.
The Java SDK can successfully submit multi-part requests to Walmart. I wrote a wrapper around the SDK functions that can accept basic command line input to read a text file and send the appropriate call with attached files. From here, you can just call the .jar file (I do it via dynamically generated batch file) from your C# program and receive responses back via text file. This is a sub-optimal system, but it works reliably and when the choice was between updating inventory on 2000 items every day and using some dirty code, I went with the Java wrapper method. This will be replaced as soon as the C# SDK comes out, but I believe this is one of the reasons why the C# SDK may be being delayed.
This solution was used, only after spending about a week trying to get boundaries / streams / attachments to work in C# and having zero success. Cases were also submitted to walmart and I was able to work with some of their top tier engineering support staff and this problem completely stumped them. I was able to trace the Java SDK execution all the way down to a built in Maven / Java function that constructed the web request so there's something under the hood that Java is doing with a multi-part request that isn't immediately clear in C#.