Sorry, I wasn't very clear about this. In levels 3 and lower of ISO-9660, files with no extension typically get a dot/period added to the end of them to signify no extension (so the entry would look like "filename.;1" or something similar). In the PS3, this additional dot seems to be ignored by default, since in the Rock-Ridge extension files with a dot at the end in the original ISO-99660 have no dot in this extension.ImgBurn can build image with files without extension, no matter what level of ISO9660 or Joliet was chosen.
I got the term "levels" from the man page of genisoimage, a fork of mkisofs. Brasero also uses that wording.