const wchar_t *contentTypeList[] = { L"application/dicom", L"multipart/related", NULL };
WinHttpOpenRequest(hConnect, L"GET", path, NULL, WINHTTP_NO_REFERER, contentTypeList, 0);
WinHttpSendRequest(...
WinHttpReceiveResponse(...
WinHttpQueryDataAvailable(hRequestHandle, &dwSize);

wchar_t *contentType = L"Content-Type: multipart/form-related; type=application/dicom";
WinHttpAddRequestHeaders(httpContext->hRequest, contentType, (unsigned long)-1, WINHTTP_ADDREQ_FLAG_ADD_IF_NEW);