/usr/share/gimp/2.0/scripts/paste-as-brush.scm is in gimp-data 2.6.12-1ubuntu1.4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75  | ; GIMP - The GNU Image Manipulation Program
; Copyright (C) 1995 Spencer Kimball and Peter Mattis
;
; script-fu-paste-as-brush
; Based on select-to-brush by Copyright (c) 1997 Adrian Likins
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 2 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program; if not, write to the Free Software
; Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
(define (script-fu-paste-as-brush name filename spacing)
  (let* ((brush-image (car (gimp-edit-paste-as-new)))
         (brush-draw 0)
         (type 0)
         (path 0))
    (if (= TRUE (car (gimp-image-is-valid brush-image)))
      (begin
        (set! brush-draw (car (gimp-image-get-active-drawable brush-image)))
        (set! type (car (gimp-drawable-type brush-draw)))
        (set! path (string-append gimp-directory
                                  "/brushes/"
                                  filename
                                  (number->string brush-image)
                                  ".gbr"))
       
        (if (= type GRAYA-IMAGE)
            (begin
                (gimp-context-push)
                (gimp-context-set-background '(255 255 255))
                (set! brush-draw (car (gimp-image-flatten brush-image)))
                (gimp-context-pop)
            )
        )
       
        (file-gbr-save RUN-NONINTERACTIVE
                       brush-image brush-draw path path
                       spacing name)
       
        (gimp-image-delete brush-image)
       
        (gimp-brushes-refresh)
        (gimp-context-set-brush name)
      )
      (gimp-message _"There is no image data in the clipboard to paste.")
    )
  )
)
(script-fu-register "script-fu-paste-as-brush"
  _"New _Brush..."
  _"Paste the clipboard contents into a new brush"
  "Michael Natterer <mitch@gimp.org>"
  "Michael Natterer"
  "2005-09-25"
  ""
  SF-STRING     _"Brush name" "My Brush"
  SF-STRING     _"File name"  "mybrush"
  SF-ADJUSTMENT _"Spacing"    '(25 0 1000 1 1 1 0)
)
(script-fu-menu-register "script-fu-paste-as-brush"
                         "<Image>/Edit/Paste as")
 |