Notepad++ excluding find filters

Doesn't look like it can be easily configured to work this way. Here are a couple of options for you

1. Use the filters dialog box to include all file extensions except the one you don't want (could get a bit long): Notepad++ Find in Files Filters
2. Figure out how to use some code to write a plugin that will do it for you: https://stackoverflow.com/questions/2188070/notepad-file-filters

Note, as of December 5th, 2019, Notepad++ 7.8.2 now supports exclude filters.

For example to exclude exe, zip and jar files, your 'find in files' filter will look like this;

*.* !*.exe !*.zip !*.jar

Relevant code change here.