This page allows to modify the different file type groups that are available in the file search and which file extensions should be assigned to which of the different groups. Each of these groups can be used as a search pattern in the custom search. The input field "Pattern" can be used as a dropdown, to select one of the available file type groups.
More information, about how to customize these groups, can be found in the chapter "File Groups" for the main module of TreeSize. Both the file search and the main module share the same file type groups, so any information that you may have gathered with the help of the Extensions view can be further analyzed with the appropriate file search filter.