If you want to read and process more than one file type then either you should leave Source name mask blank or use a mask of *.*, the listener selects every file in the directory identified in the Source location field (skipping over any files that are themselves directories) else you have to create one source name mask for each file listener. any way your not interested in second approach then you can go for first approach there is no problem and these are the only ways to read different file types.
Based on the previous responses, it doesn't look like that is currently possible. If you'd like, an enhancement request can be raised on your behalf for this to be considered for inclusion in future releases.