What site did you download the firmware from and what browser did you use? I found that downloading zip files using Firefox from specific D-Link site's strip the zip extension, and I have to manually add the extension after downloading to extract the file.
Thanks for replying.
I got the link from ftp://ftp.dlink.eu/Products/dns/dns-320/driver_software/DNS-320_fw_revA1A2_2-03b03_all_en_20121005.zip
and the zip contains 3 files: one with no extension, the release note in doc format and a changes.txt file.
I do use firefox but I don't think the zip file is corrupted since I can see the files inside.
I extract all files to a folder on my local box and then select the file with no extension in the firmware upgrade web ui. After clicking "Apply" I get the message mentioned above. I also tried selecting the zip file in the web ui but got the same result.
Thanks