Determine whether a field references files stored in {file_managed}.
array $field: A field array.
The field column if the field references {file_managed}.fid, typically fid, FALSE if it doesn't.
function file_field_find_file_reference_column($field) {
foreach ($field['foreign keys'] as $data) {
if ($data['table'] == 'file_managed') {
foreach ($data['columns'] as $field_column => $column) {
if ($column == 'fid') {
return $field_column;
}
}
}
}
return FALSE;
}