Hi there I know that there was an issue for this earlier #65 and it was dismissed, I do however think that using the iOS convention doesn't help if the convention doesn't work with different web based systems. iOS is not webbased and thus having an @ in the file name isn't a problem.
Google Appengine does not even allow files containg @ to be uploaded so we patched our own version of Photoswipe to not contain the @ sign.
Since it is only one file. I believe it would be better to change the @ to a dash. Conventions are there to make our life easier not to make it harder, and if you have only 1 file that uses a convention, it would make sense from a compatibility point of view to just change the file name to support both Google Appengine and the ezpublish CMS from the previous issue.
Pull request for issue is in #434
@2x is the convention for denoting pixel density, due to the fact that Apple went with it first. Obviously when Apple started using it, they were not using it for the web, however...
Hence, '@2x' is a completely valid & suitable file name for use on the web. That, combined with it being the recognised standard, means that we will not make this change.
If Google AppEngine doesn't conform to everything allowed by the RFC on URIs, I'm afraid that's something you should probably take up with them.