This suggestion is related to this discussion in the development forum. While I was on the Moxiecode website I had a look at plupload, then checked how XenForo attachments are managed in the attachment_editor script. Plupload events should allow (I think, but I haven't tested it) to keep a quite similar structure with the advantage to have several "runtimes", especially html5 et flash. Since their OEM license is at 80 euros, it looks interesting and affordable.
Would definitely like to see a built-in option to upload multiple photo's at once in future Xenforo releases, my site is image intensive and users often upload between 10 and 20 pics at a time, a lot of them being mobile users too.
Given Flash's continued demise and the likely long path for people to upgrade to XF2 which has an HTML5 uploader, we've now backported a similar approach to 1.5. This has now been rolled out here. You shouldn't really see any significant differences on desktop browsers; mobile browsers will now support multiple file uploads though.
A change like this isn't normally something we'd do within a bug fix release (it also changes some of the APIs exposed by the JS), so while we have enabled the HTML5 uploader system by default, it can be disabled if needed. If add-ons load the attachment editor JS directly in their own templates, they will need changes to load the new file instead; if they don't, they will continue to use the Flash uploader.