WordPress at a glance
filter is not described

ext2type filter-hook . WP 2.5.0

Filters file type based on the extension name.

Usage

add_filter( 'ext2type', 'filter_function_name_1298' );
function filter_function_name_1298( $ext2type ){
	// filter...

	return $ext2type;
}
$ext2type(array)
Multi-dimensional array with extensions for a default set of file types.

Where the hook is called

wp_get_ext_types()
wp-includes/functions.php 2558-2568
return apply_filters( 'ext2type', array(
	'image'       => array( 'jpg', 'jpeg', 'jpe',  'gif',  'png',  'bmp',   'tif',  'tiff', 'ico' ),
	'audio'       => array( 'aac', 'ac3',  'aif',  'aiff', 'flac', 'm3a',  'm4a',   'm4b',  'mka',  'mp1',  'mp2',  'mp3', 'ogg', 'oga', 'ram', 'wav', 'wma' ),
	'video'       => array( '3g2',  '3gp', '3gpp', 'asf', 'avi',  'divx', 'dv',   'flv',  'm4v',   'mkv',  'mov',  'mp4',  'mpeg', 'mpg', 'mpv', 'ogm', 'ogv', 'qt',  'rm', 'vob', 'wmv' ),
	'document'    => array( 'doc', 'docx', 'docm', 'dotm', 'odt',  'pages', 'pdf',  'xps',  'oxps', 'rtf',  'wp', 'wpd', 'psd', 'xcf' ),
	'spreadsheet' => array( 'numbers',     'ods',  'xls',  'xlsx', 'xlsm',  'xlsb' ),
	'interactive' => array( 'swf', 'key',  'ppt',  'pptx', 'pptm', 'pps',   'ppsx', 'ppsm', 'sldx', 'sldm', 'odp' ),
	'text'        => array( 'asc', 'csv',  'tsv',  'txt' ),
	'archive'     => array( 'bz2', 'cab',  'dmg',  'gz',   'rar',  'sea',   'sit',  'sqx',  'tar',  'tgz',  'zip', '7z' ),
	'code'        => array( 'css', 'htm',  'html', 'php',  'js' ),
) );

Where the hook is used (in WP core)

Does not used.