Update* After further testing, this problem only occurred at my work computer. When I accessed my AGOL account from a different machine, i was able to open the attachments fine. This must be an issue with my computer not recognizing the attachment without the file extension as a .jpg.
In this instance, I am using iOS on an ipad air. Here is what I have discovered:
When I collect an attachment in collector from ipad when online, it names the attachment Photo1.jpg, Photo2.jpg, etc.
When I do the same in offline mode the attachment is named Photo 1, Photo 2, etc. with no file extension
When I collect on my android device it names it consistently both online and offline but with a completely different naming schema something like MapName_169_0_ArcGISApp_110990398400934.jpg.
This brings up the issue that depending on which device and what mode you are collecting attachments, the result will be different file naming schemas and in the case of the offline ipad, a file without an extension. I would imagine that my work computer is not the only one out there that will have difficulty recognizing a file without an extension.
I do not see any way that I can define the name of the attachments which would alleviate this problem. I had posted a question about pulling attachments out of the application on a different post. If I have attachments for 500 features and each has a photo called Photo1.jpg, this could cause some headaches.
It would be nice to know if others are seeing this same behavior on their mobile devices.