How to upload files to the sites/default/files place without going to the Cpanel backend just through admin backend?
If you want to upload files from UI you can use the elFinder file manager module. I've checked this module. Seems it's what you're looking for.
You can upload multiple files from admin UI itself. You can access the URL like follows

http://example.com/elfinder


Features


  Integrated with WYSIWYG API  capable editors: CKEditor, FCKeditor, TinyMCE, BUEditor (D6), jWYSIWYG (D6), YUI, WYMeditor
  Multiple file upload
  File copying/moving with Drag &amp; Drop support
  Quick file and directory rename
  Image resizing
  Lightbox-preview
  Contect menu with generic file/directory operations
  Inline text file editing
  Favories directory list
  Directory selection history
  Working with archives
  File descriptions
  


screenshot