Put the extension (no dot) in the Ext entry for your UDL

That was also explained in the UDL documentation already linked.

If you have an extension (like .log) that is common to all your logfiles, you can even use the UDL definition to associate that extension with your defined UDL, so Notepad++ will automatically format files with that extension using your UDL.

Thanks Peter, this is helpful. Do you have a link or specific instructions on how to do this?

That sounds like a job for User Defined Languages (UDL). Basically, add keywords to the various groups when defining your language, and each of those keyword groups can have their own style setting.

If you have an extension (like .log) that is common to all your logfiles, you can even use the UDL definition to associate that extension with your defined UDL, so Notepad++ will automatically format files with that extension using your UDL.

More information on UDL:

• UDL Overview = https://npp-user-manual.org/docs/user-defined-language-system/
• UDL Details = https://ivan-radic.github.io/udl-documentation/
• Collection of pre-made UDL definitions = https://github.com/notepad-plus-plus/userDefinedLanguages

If you have specific questions, feel free to ask.