You may want to try the Report.Version table activity in CatTools to see whether this returns the information you require. There 'Image' column in this report should show the filename of the flash content. http://www.kiwisyslog.com/help/cattools/act_repversion.htm
Alternatively, if its possible to get the flash information via SNMP, then you could try the Report.SNMP.System Summary activity which allows you to specify 5 custom OIDs. http://www.kiwisyslog.com/help/cattools/act_repsnmpsyssumry.htm
If neither of these work, then a custom activity would be the way to go.