The SourceURL link in the mediaInfos property of the PopupTemplate does not go through a proxy.
Bug ID Number
January 7, 2016
December 18, 2020
Operating System Version
7.0 64 Bit
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Looks to be similar to BUG - 000089862. Please look at the comment and description provided for that issue as it appears to be the same customer.
Set SourceURL is as follows:
http://<path to proxy.ashx>?<path to secured service attachment>