It looks like you are attempting to use an application/x-www-form-urlencoded Request, but are using the content-type 'multipart/form-data'. If you make the following changes, your code seemed to work properly for me:
1. Change:
request.ContentType = "multipart/form-data";
to:
request.ContentType = "application/x-www-form-urlencoded";
2. Change:
data.Append("&file=" & HttpUtility.UrlEncode(CSVFile))
to:
data.Append("&data=" & HttpUtility.UrlEncode(CSVFile))
If you are still receiving 400 errors after that, then please send us a few lines of example data from your CSV so that we can see exactly what information you are using.
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