[vs2010+] Handle compileas for files with "unknown" extensions.

modules/vstudio/tests/vc2010/test_files.lua

@@ -657,6 +657,18 @@
 		]]
 	end
 
+	function suite.onCompileAsExt()
+		files { "hello.unknown_ext" }
+		filter "files:hello.unknown_ext"
+			compileas "C++"
+		prepare()
+		test.capture [[
+<ItemGroup>
+	<ClCompile Include="hello.unknown_ext">
+		<CompileAs>CompileAsCpp</CompileAs>
+		]]
+	end
+
 --
 -- Check handling of per-file cdialect.
 --

modules/vstudio/vs2010_vcxproj.lua

@@ -1360,6 +1360,10 @@
 				if fcfg.buildaction then
 					return m.categories[fcfg.buildaction] or m.categories.None
 				end
+
+				if fcfg.compileas ~= nil and fcfg.compileas ~= "Default" then
+					return m.categories.ClCompile
+				end
 			end
 		end