While '.jpeg' and '.jpg' are interchangeable, they mean the same thing...Joint Photographic Experts Group...the difference here is that the forum software only recognizes exact matches when allowing attachments. IPhones, and probably other smart devices, use '.jpeg' as the file name extension which is not on the list of Simple Machines' accepted extensions - at least for our version.
So, until an upgrade is done, you'll need to rename the file extension from '.jpeg' to '.jpg' before attempting to add the attachment.