not that its a GOOD answer, but, what I've found that works is this
*.sql ; *.asp ; *.html ; *.as ; *.mxml
Its definitely not an 'exclude' list, but, by including all the types I need to look at, I get the same result. In my case, I've only got those handfuls of types that I look for often. If your set was wider, this would become even less useful.
There may be a more elegant way, but you could use PowerShell to get all of the extensions in a folder, and then just remove the ones you don't want. This example gets all of the extensions in the "C:\Program Files\Internet Explorer" folder.
NB: don't forget to surround (Get-ChildItem ... -join ";*" with parentheses.
I got a tip from another forum that helped me solve this. Mark the folders you want to exclude as 'hidden'. By default the 'Find in Files' in N++ excludes hidden directories. You can then revert the hidden directories when you're done.
All my results where on M:, you will have to adapt the start to what fits your results. If you run a "Replace all" with an empty string it removes all results in files ending with "csproj" or "dll", and also in the files "packages.config" and "Build.props".
Note, as of December 5th, 2019, Notepad++ 7.8.2 now supports exclude filters. The help documentation describes exclude filters in the Find in Files tab section.
For example to exclude exe, zip and jar files, your 'find in files' filter will look like this;