Hi,
It could be possible that the link is being processed in the background (to generate a preview).
The best for the moment would be to increase the download limit.
We do not have any iphone in the team so it will be difficult for us to perform tests, reproduce the problem and analyse the server logs.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.