Thanks thanos1983, I tried tcpdump and Wireshark, but the content of the post requests looks good and the multi-part attachments are there, but somehow it is not being parsed.

I also see this in apache error log: 
[Thu Mar 14 18:40:04 2019] [error] [client 17.108.244.6] (70008)Partia
+l results are valid but processing is incomplete: ap_get_brigade fail
+ed during prefetch
[Thu Mar 14 18:40:04 2019] [error] Internal apreq error

[download]
Will update here if I find anything.

In reply to Re^2: Apache2::Request Internal apreq error by evergreen
in thread Apache2::Request Internal apreq error by evergreen

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