Permitted upload file extensions default to 'txt'.

Currently the permitted upload file extensions defaults to 'txt', which doesn't make a whole lot of sense for an image field. (Looks like this is set by filefield). In fact, leaving it as the default txt causes an error when submitting the form since imagefield only allows extensions of the type jpg, jpeg, png, and gif.

The following patch adds 'jpg jpeg png gif' as the default extensions.