There are several bugs in the patch

1. Omitting the filetype doesn't work:

Traceback (most recent call last):
  File "./sk2ppm.py", line 127, in ?
    result = main()
  File "./sk2ppm.py", line 123, in main
    export_raster(context, ppmfile, resolution, use_bbox, format = format,
UnboundLocalError: local variable 'format' referenced before assignment


2. The filelist in Script.export_raster is only meant for the tk file dialog.  The patch uses the file name extension from that list as the format parameter to export_raster.  That parameter is not the extension.  It's the name PIL uses for the format which can be different.  In the case of JPG for instance the extension is "jpg" whereas the PIL format name is "JPEG" (PIL matches the format name case insensitivley, though).  This means -f jpg gives an error:

Traceback (most recent call last):
  File "./sk2ppm.py", line 127, in ?
    result = main()
  File "./sk2ppm.py", line 124, in main
    antialias = alpha)
  File "./Script/export_raster.py", line 163, in export_raster
    image.save(filename, format = format)
  File "/usr/local/lib/python2.2/site-packages/PIL/Image.py", line 729, in save
    SAVE[string.upper(format)](self, fp, filename)
KeyError: JPG