data_transfer: directly pass the buffer

Change-Id: I62f93b4b0706863debc9595b0b4646c3b0974b52

src/data_transfer.cpp

@@ -835,7 +835,7 @@ IncomingFile::process()
     channel_->setOnRecv([w = weak()](const uint8_t* buf, size_t len) {
         if (auto shared = w.lock()) {
             if (shared->stream_.is_open())
-                shared->stream_ << std::string_view((const char*) buf, len);
+                shared->stream_.write(reinterpret_cast<const char*>(buf), len);
             shared->info_.bytesProgress = shared->stream_.tellp();
         }
         return len;