Seems the file dialogue has changed from Max 5 to Max 6.
Using the same patch to open sample files, I notice that Max 6 will only allow me to open files that have a file suffix (.aiff, .wav etc.), while Max 5 allows me to open sample files even if they have not suffix.
First screen shot is from "open" in a patch in Max 5, second in Max 6. All files are greyed out in the latter.