WordPress at a glance
filter is not described

upload_mimes filter-hook . WP 2.0.0

Filters list of allowed mime types and file extensions.

Usage

add_filter( 'upload_mimes', 'filter_function_name_4800', 10, 2 );
function filter_function_name_4800( $t, $user ){
	// filter...

	return $t;
}
$t(array)
Mime types keyed by the file extension regex corresponding to those types. 'swf' and 'exe' removed from full list. 'htm|html' also removed depending on '$user' capabilities.
$user(int/WP_User/null)
User ID, User object or null if not provided (indicates current user).

Where the hook is called

get_allowed_mime_types()
wp-includes/functions.php 2636
return apply_filters( 'upload_mimes', $t, $user );

Where the hook is used (in WP core)

wp-includes/ms-default-filters.php 71
add_filter( 'upload_mimes', 'check_upload_mimes' );