Rename JSX files with proper .jsx extension (#23733)

app/javascript/mastodon/features/compose/components/character_counter.jsx

@@ -0,0 +1,25 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { length } from 'stringz';
+
+export default class CharacterCounter extends React.PureComponent {
+
+  static propTypes = {
+    text: PropTypes.string.isRequired,
+    max: PropTypes.number.isRequired,
+  };
+
+  checkRemainingText (diff) {
+    if (diff < 0) {
+      return <span className='character-counter character-counter--over'>{diff}</span>;
+    }
+
+    return <span className='character-counter'>{diff}</span>;
+  }
+
+  render () {
+    const diff = this.props.max - length(this.props.text);
+    return this.checkRemainingText(diff);
+  }
+
+}