How should sorting work for altered columns?

One approach would be to add another parameter to hook_file_metadata_table_metadata(), indicating whether a field should be considered numeric or string-based.

So instead of:

function hook_file_metadata_table_metadata() {
  return array(
  'cid' => 'Comment ID'
  );
}

We could use:

function hook_file_metadata_table_metadata() {
  return array(
  'cid' => array(
    'title' => 'Comment ID',
    'sort' => 'numeric',
  );
}

This would require minor tweaks to file_metadata_table_metadata() to have it store the sort parameters as well ... in which case I would rename the function file_metadata_table_types(), and create a new file_metadata_table_metadata() wrapper function which returns only $key => $type['title'] (identical structure to the existing function today).

After these changes, the sort function could be modified as follows:

  $metadata = file_metadata_table_types();
  if ($metadata[$on]['sort'] == 'numeric') {
    $function = "(\$a['{$on}'] > \$b['{$on}']);";
  }
  // For other fields, sort by string value.
  else {
    $function = "strcmp(\$a['{$on}'],\$b['{$on}']);";
  }

Implemented as described above.

Committed as b71041f.

Module name change.