Does anyone have any ideas or clues how to fix this issue?
It's bizarre. In Firefox 39, if you open a photo attachment and right-click over the image and choose "Save Image as..." you get attachment.php as the file name. This is the wrong/unexpected behavior.
In Firefox 39, if you open a photo attachment and right-click outside the image area (not directly over the photo) and choose "Save Page As..." you get the correct file name and extension.
This is just backward. You would think the page might be called attachment.php since that's the script serving the image, but the actual image should save with the correct file name.
It really doesn't matter is it's a bug in Firefox or a bug in vBulletin, either way I still need to find a work around.
Thanks for any help or suggestions.