Implements hook_file_mimetype_mapping_alter().
function file_test_file_mimetype_mapping_alter(&$mapping) {
// Add new mappings.
$mapping['mimetypes']['file_test_mimetype_1'] = 'madeup/file_test_1';
$mapping['mimetypes']['file_test_mimetype_2'] = 'madeup/file_test_2';
$mapping['mimetypes']['file_test_mimetype_3'] = 'madeup/doc';
$mapping['extensions']['file_test_1'] = 'file_test_mimetype_1';
$mapping['extensions']['file_test_2'] = 'file_test_mimetype_2';
$mapping['extensions']['file_test_3'] = 'file_test_mimetype_2';
// Override existing mapping.
$mapping['extensions']['doc'] = 'file_test_mimetype_3';
}