通过从 MultipartFormDataContent 中移除标头并在没有验证的情况下重新添加它解决了这个问题:

Solved this by removing the header from MultipartFormDataContent and re-adding it back without validation:

content.Headers.Remove("Content-Type");
content.Headers.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=" + boundary);