Leave .css files un-transformed

Projects using PostCSS may have enabled Sass-incompatible plugins
which can result in .css files that cannot be parsed by the Sass
compiler.

Since PostCSS is able to extract nested selectors without any kind
of transformation, we can pass in the source as-is.

Author: jgoz

src/helpers/__tests__/fixtures/test.module.css

@@ -11,6 +11,7 @@
 }
 
 .class_d {
+  @mixin thisBreaksSass;
   color: rebeccapurple;
 }
 

src/helpers/cssSnapshots.ts

@@ -23,10 +23,15 @@ const flattenClassNames = (
 export const enum FileTypes {
   scss = 'scss',
   less = 'less',
+  css = 'css',
 }
 
 export const getFileType = (fileName: string) =>
-  fileName.endsWith('less') ? FileTypes.less : FileTypes.scss;
+  fileName.endsWith('.css')
+    ? FileTypes.css
+    : fileName.endsWith('.less')
+    ? FileTypes.less
+    : FileTypes.scss;
 
 export const getClasses = (css: string, fileType: FileTypes) => {
   try {
@@ -36,8 +41,10 @@ export const getClasses = (css: string, fileType: FileTypes) => {
       less.render(css, { asyncImport: true } as any, (err, output) => {
         transformedCss = output.css.toString();
       });
-    } else {
+    } else if (fileType === FileTypes.scss) {
       transformedCss = sass.renderSync({ data: css }).css.toString();
+    } else {
+      transformedCss = css;
     }
 
     const processedCss = processor.process(transformedCss);