Multipart form upload prepends the file

--<file-hash>
Content-Disposition: form-data; name=file; filename=<file-name>;
filename*=utf-8''<file-name>
{
...
}

HttpClient client = new();
MutlipartFormDataContent formData = [];
FileStream fileStream = new("filepath", FileMode.Open, FileAccess.Read);
StreamContent fileContent = new(fileStream);
formData.Add(fileContent, "file", "filename.csv");
client.PutAsync("url", formData);