diff --git a/app/lib/language_utils.rb b/app/lib/language_utils.rb
index 46a0941a7..2425e1b34 100644
--- a/app/lib/language_utils.rb
+++ b/app/lib/language_utils.rb
@@ -6,7 +6,20 @@ def self.data
       base = YAML.load_file(Rails.root.join("config/languages.yml"))
       custom_path = Rails.root.join("config/languages_custom.yml")
       custom = File.exist?(custom_path) ? YAML.load_file(custom_path) : {}
-      base.deep_merge(custom)
+      merged = base.deep_merge(custom) { |_key, base_val, custom_val|
+        base_val.is_a?(Array) && custom_val.is_a?(Array) ? base_val | custom_val : custom_val
+      }
+      custom.each do |name, info|
+        next unless info.key?("extensions")
+        merged_extensions = merged.dig(name, "extensions") || []
+        merged_extensions.each do |ext|
+          merged.each do |other_name, other_info|
+            next if other_name == name
+            other_info["extensions"]&.delete(ext)
+          end
+        end
+      end
+      merged
     end
   end
 
diff --git a/config/languages_custom.yml b/config/languages_custom.yml
index e7de2d090..b4a66754c 100644
--- a/config/languages_custom.yml
+++ b/config/languages_custom.yml
@@ -11,3 +11,9 @@ Lapse:
 AsciiDoc:
   extensions:
   - ".ad"
+
+Assembly:
+  extensions:
+  - ".asm"
+  - ".s"
+  - ".S"  
\ No newline at end of file
diff --git a/test/lib/language_utils_test.rb b/test/lib/language_utils_test.rb
new file mode 100644
index 000000000..8d5f9b9db
--- /dev/null
+++ b/test/lib/language_utils_test.rb
@@ -0,0 +1,33 @@
+require "test_helper"
+
+class LanguageUtilsTest < Minitest::Test
+  def setup
+    LanguageUtils.instance_variable_set(:@data, nil)
+    LanguageUtils.instance_variable_set(:@extension_map, nil)
+    LanguageUtils.instance_variable_set(:@alias_map, nil)
+    LanguageUtils.instance_variable_set(:@filename_map, nil)
+  end
+
+  def teardown
+    LanguageUtils.instance_variable_set(:@data, nil)
+    LanguageUtils.instance_variable_set(:@extension_map, nil)
+    LanguageUtils.instance_variable_set(:@alias_map, nil)
+    LanguageUtils.instance_variable_set(:@filename_map, nil)
+  end
+
+  def test_custom_assembly_extensions_override_other_languages
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("foo.asm")
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("foo.a51")
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("foo.nasm")
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("foo.s")
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("foo.S")
+  end
+
+  def test_custom_language_additions
+    assert_equal "AsciiDoc", LanguageUtils.detect_from_extension("foo.ad")
+  end
+
+  def test_custom_language_without_extension_conflict
+    assert_equal "Lapse", LanguageUtils.find_name("Lapse")
+  end
+end