The AAE file: What is its purpose, how do I open it and can it be deleted?
Many iPhone, Windows and Apple Mac users sorting through photo libraries have probably noticed the appearance of AAE files, ending in the .AAE extension. By their file names, these typically look like image files (eg IMG_2123.AAE) but if you try to open them, you'll see an error message saying they are not valid image files. Windows users will see a message saying 'Windows cannot open this file' and a suggestion to search the web for something that can. This is especially confusing for Windows users as by default, Windows hides extensions, so in these cases an AAE file will simply look like IMG_2123 with a generic blank icon.
You may have also noticed that files you edited on your iPhone (or Mac) using the Photos app now appear unedited when you transfer them from your iPhone to another computer or software program on the Apple Mac. So what's going on?
In short, editing in the Photos App from iPhone release iOS 8 and above does not change the original image file. In the past editing a file would over-write the original. The iOS 8 and above leave the original photograph alone and saves your edit instructions in a separate file, namely the AAE file (also called the AAE sidecar file). The location of the AAE file is in the same folder as the original photo and follows the same naming format, but instead of .JPG at the end, it will have the .AAE file extension. For example IMG_2123.JPG that has been edited will have another file created called IMG_2123.AAE. This may explain why at first glance the appearance of AAE files is somewhat random: it only appears if you edit an image file.
The implications of this are important. Firstly for most users on Windows or older OS X versions of a Mac (pre-OS X Yosemite) it means that when you copy across the photographs, the alterations won't be applied. In order to apply your edits once the file has been moved, you will need software that can read and apply the AAE file. Unfortunately very few options exist for this, none are apparently available outside of the Apple Mac or iPhone universe. But there is another solution if you have the photos and the AAE files still on your iPhone (you can see this if you look at the photograph on your iPhone and the edits are still there). To get the edited photos out, you will need to do something that causes your phone to render the final edit. One of the easiest techniques is to email yourself the photos from within your phone. You can also post the edited version on Facebook or Instagram, but be aware that these options will drop the quality of the photos quite noticeably. Saving the photos to the iCloud is one solution, likewise Airdrop but it is a tricky process to implement on a Windows PC.
Should I delete or keep the AAE files?
Currently AAE files on Windows serve very little purpose as they can't yet be used, but this may change down the track. So you may need to clean up the AAE files manually (ie delete them). Alternatively since they are so small (~1kb), you could leave them. But be aware that if you rename your photos or move them to a folder that is different to the AAE file, the connection to the AAE file will be lost - they must be in the same folder and have the same root file name, with only the extension changing (eg changing IMG_2123.JPG to file Hawaii001.JPG will need to have the AAE file changed to Hawaii001.AAE to keep the files in sync). The difficulty of course is knowing which AAE file belongs to what photo once the names of the original photos are changed.
Some final thoughts
The idea of being able to edit an image file and leave the original intact is actually a good idea in some cases, especially if you make a mistake or want to go back and change the edit settings. Unfortunately despite the AAE file being written in XML format, few options are available for those using Windows, earlier versions of OS X or sharing with Android phones. In the vast majority of times, people are happy with edits and won't ever go back to change photos. So littering hard drives with useless files and making people double up on files is a step backwards. A simpler solution for Apple would be giving users the option of applying changes to the original photo or saving a new edited copy in the edit window of the Photos App, or when exiting edit mode.