Propagates the extensions overrides provided by CLI during files walk (#8668)

* Propagates the extensions overrides provided by CLI during files walk

* Adds tests for issue #7620, PR #8668

packages/babel-cli/src/babel/file.js

@@ -137,7 +137,11 @@ export default async function({ cliOptions, babelOptions }) {
         const dirname = filename;
 
         util
-          .readdirForCompilable(filename, cliOptions.includeDotfiles)
+          .readdirForCompilable(
+            filename,
+            cliOptions.includeDotfiles,
+            cliOptions.extensions,
+          )
           .forEach(function(filename) {
             _filenames.push(path.join(dirname, filename));
           });

packages/babel-cli/src/babel/util.js

@@ -29,8 +29,11 @@ export function readdir(
 export function readdirForCompilable(
   dirname: string,
   includeDotfiles: boolean,
+  altExts?: Array<string>,
 ) {
-  return readdir(dirname, includeDotfiles, isCompilableExtension);
+  return readdir(dirname, includeDotfiles, function(filename) {
+    return isCompilableExtension(filename, altExts);
+  });
 }
 
 /**