Don’t treat .7z as part of a version.

Library/Homebrew/extend/pathname.rb

@@ -200,7 +200,7 @@ class Pathname
     return archive_ext if archive_ext
 
     # Don't treat version numbers as extname.
-    return "" if basename.match?(/\b\d+\.\d+[^\.]*\Z/)
+    return "" if basename.match?(/\b\d+\.\d+[^\.]*\Z/) && !basename.end_with?(".7z")
 
     File.extname(basename)
   end

Library/Homebrew/test/pathname_spec.rb

@@ -137,6 +137,11 @@ describe Pathname do
     it "does not treat version numbers as extensions" do
       expect(described_class.new("foo-0.1").extname).to eq("")
       expect(described_class.new("foo-1.0-rc1").extname).to eq("")
+      expect(described_class.new("foo-1.2.3").extname).to eq ""
+    end
+
+    it "supports `.7z` with version numbers" do
+      expect(described_class.new("snap7-full-1.4.2.7z").extname).to eq ".7z"
     end
   end