Multi-value formatters + single-value record + Views = Fail

I answer your verbose (and excellent) report with equal verbosity. ;)

I don't think FileField's sanitize function is at fault for this problem, it looks like we're somehow not following the documentation for passing in parameters into content_format().

Like you said at first, with either a multivalue formatter or a single value formatter, the $item passed into content_format() is the same, a single file object. However, according to the PHP doc at the top of content_format() the $item variable should already be an array of items if the destination formatter is multivalue:

/**
 * @param $item
 *   The field item(s) to be formatted (such as $node->field_foo[0],
 *   or $node->field_foo if the formatter handles multiple values itself)
 */

So $item should already be an array of $file objects, not a single file, and filefield_field_sanitize() shouldn't need to be changed at all.

Now we get into the Views integration. We need to know why CCK isn't getting an array of items and instead is only getting a single item. CCK already has a Views handler "content_handler_field_multiple" which properly calls content_format() with an array of items. My guess is that FileField needs to tell CCK (to tell Views) that FileFields should use the content_handler_field_multiple handler instead of the normal content_handler_field.

That's as far as I can discover currently, I'll take a look at FileField's filefield_field_settings_views_data() function to see if any adjustment needs to be made there.

Well, upon further inspection I actually couldn't find anything wrong. In my installation (CCK 2.2, Views 2.3, FileField 3.x dev), everything does receive the proper parameters.

Setup:
- Create a new content type
- Add a multivalue FileField to the type
- Add some nodes with files attached
- Make a new view, add the "Content: [Label of Field] (field_[name_of_field])" field
- Ensure the "Group multiple values" checkbox is checked for this field. This option only shows up if the CCK field is multivalue. See screenshot.
- Select a multivalue formatter from the "Format" drop-down.

I didn't have any multivalue formatters around on my local, so I modified the default file formatter to be multivalue:

// In filefield.module:
function filefield_field_formatter_info() {
  return array(
    'default' => array(
      'label' => t('Generic files'),
      'field types' => array('filefield'),
      'multiple values' => CONTENT_HANDLE_MODULE,
      'description' => t('Displays all kinds of files with an icon and a linked file description.'),
    ),
  );

// In filefield_formatter.inc:
function theme_filefield_formatter_default($element) {
  $field = content_fields($element['#field_name']);

  $output = '';
  foreach (element_children($element) as $delta) {
    $file = $element[$delta]['#item'];
    $output .= theme('filefield_item', $file, $field);
  }
  return $output;
}

Then everything worked as expected. Multiple $file objects were passed into the $item parameter of content_format(), then filefield_field_sanitize(), then finally theme_filefield_formatter_default(). As far as I can tell, everything is working properly.

Does the attached patch fix it for you ?

Cool, committed. Thanks for the detective work !

Using Drupal Core 6.20 and CCK 6.x-2.9 I have repeatedly hit this problem: Only the first field of a Multi-value item is output.
The problem persists even if I disable Devel and Themer, and flush all caches.

More precisely I should say - my problem matches this description perfectly, but it may be some other cause. The missing multi-fields are truly missing, as verified by a print_r to disk.

Here's the catch though - this only seems to happen in one content type or one view, once in a while (actually, this is the only content which utilizes multi-value, so it's not clear if it could be systemwide or if there is something "special" about this content and view, which was originally created around March 2009 using older Drupal modules, and the system since upgraded of course).

I am not sure what triggers this bad state for this content, but guess what fixes it for me?

If I change the field label from "Widget Label (fieldname)" to any of the other 2 choices, then save that field, and finally save the view itself. That fixes a particular field's display. (To completely fix all affected fields, I need to repeat the field-label "bump" on all of the fields, then save the View.)

Again, after "bumping" the fields' labels in Views Fields, then Saving the field, the content view will show it is repaired. All instances of the fields display.

Post CCK 2.2 you can still get a problem that has a similar nature.
In any case, if someone else hits this, try my workaround, and if it fixes it for you, please file a new issue.
(I suspect it is just my content or View having become corrupt, and so I'm not sure there is a real issue to file).