Index: wp-admin/js/theme-plugin-editor.js
===================================================================
--- wp-admin/js/theme-plugin-editor.js	(revision 42426)
+++ wp-admin/js/theme-plugin-editor.js	(working copy)
@@ -73,9 +73,37 @@
 			}
 			return undefined;
 		} );
+
+		$( document ).on( 'keyup', '#plugin-files-search, #theme-files-search', component._searchFile );
 	};
 
 	/**
+	 * Search file.
+	 *
+	 * @since 4.9.2
+	 * @returns {void}
+	 */
+	component._searchFile = function() {
+		var parent      = $( '#templateside' );
+		var directories = parent.find( 'ul ul li' );
+		var files       = parent.find( 'ul ul li a' );
+
+		// Expand all directories.
+		directories.attr( 'aria-expanded', true );
+
+		// Hide Directories. Becuase, Below we show those directories
+		// which have search term in the file name.
+		directories.hide();
+
+		// Search file and ONLY show these directories which "contain" the file name.
+		var rex = new RegExp( $( this ).val(), 'i' );
+		files.filter( function() {
+			var file_name = $.trim( $( this ).text() ) || '';
+			return rex.test( file_name );
+		} ).parents( 'li' ).show();
+	};
+	
+	/**
 	 * Set up and display the warning modal.
 	 *
 	 * @since 4.9.0
Index: wp-admin/plugin-editor.php
===================================================================
--- wp-admin/plugin-editor.php	(revision 42426)
+++ wp-admin/plugin-editor.php	(working copy)
@@ -239,6 +239,7 @@
 
 <div id="templateside">
 	<h2 id="plugin-files-label"><?php _e( 'Plugin Files' ); ?></h2>
+	<input id="plugin-files-search" type="text" placeholder="<?php _e( 'Search file..' ); ?>">
 
 	<?php
 	$plugin_editable_files = array();
Index: wp-admin/theme-editor.php
===================================================================
--- wp-admin/theme-editor.php	(revision 42426)
+++ wp-admin/theme-editor.php	(working copy)
@@ -243,6 +243,7 @@
 ?>
 <div id="templateside">
 	<h2 id="theme-files-label"><?php _e( 'Theme Files' ); ?></h2>
+	<input id="theme-files-search" type="text" placeholder="<?php _e( 'Search file..' ); ?>">
 	<ul role="tree" aria-labelledby="theme-files-label">
 		<?php if ( ( $has_templates || $theme->parent() ) && $theme->parent() ) : ?>
 			<li class="howto">