To reproduce: - Go to a local file directory on your phone, e.g. "file:///home/user/MyDocs/" on the N900. Notice how directories have a 'folder' icon, but all the files with the various extensions don't have a 'file' icon. Instead they have the replacement 'File:' text. Note that this works fine on Fennec on Windows. I see this occuring on the N900 and Droid.
mfinkle says it has something to do with mozicon: