WordPress already allows you to easily make quick changes to your themes or plugins without having to use a FTP client. To get a more organized view of your files, you may want to install File Manager Plugin For WordPress. You will be able to view files & folders and interact with them via drag & drop. Multiples theme are included too.
Thanks to this script, you can view and edit files like a pro. Want to view PDF files or make quick changes to your images? No problem. Webmasters are in control of who can access what. File Manager lets you upload files, create archives, and perform cut/copy/paste operations.