Support file and image fields in D7's devel_generate

Replace our upload.module support with support for file and image fields. In the end, I expect to see no UI for this on the generate form, just like other Fields.

The code for this should be defined in devel_generate.fields.inc. See the functions there for examples.

The traditional image to use when no default has been specified is a random png from the misc dir with this code:

  $files = file_scan_directory(DRUPAL_ROOT. '/misc', '/^.*\.png$/');
  $source = $files[array_rand($files)];