Comment 17 for bug 524065

Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2) Gecko/20100218 Ubuntu/9.10 (karmic) Firefox/3.6
This was confirmed on trunk recently (see LP bug). Apparently this isn't happening on Windows Vista with 3.6 (See LP Bug). I get this in 3.6 for images and compressed file formats.
As of now in my download list, I have with missing icons:
.tiff
.xml
.zip
.bz2
.png
.txt
.jpg

But, the following do have icons:
.pdf
.gz
.ico
.html
.patch
.asp
.jsp
.sh