I have enjoyed using your well-considered plugin, up until recently.
I am currently using the latest version of WordPress v3.4.2, and the latest All-in-One-Favicon plugin v4.0, on the latest version of Mac Mountain Lion v10.8.2... however, I am not able to upload any files (ico, png, gif) on any Firefox, Chrome or Safari browser. I am only able to upload files on Windows. Curiously, Windows seems to be the only place I could find where this important feature works.
Screenshots of the "grayed-out" file names are shown in the image URLs below. (Note the darker color for clickable folder items vs. the .ico, .png & .gif files)
I took a look at the code, although I wasn't able to spot the issue, and am hoping that you can make a little time to fix... Thank you for all your efforts. They are much appreciated!!