How can I link a CSS file only on single posts?

Question

If I understand is_single() correctly, it's only supposed to return true when the current "post" (in a generic sense) is a literal post (i.e., it has a post_type of post), and that it will return false when the current "post" is a page, attachment, etc. This is what the Codex says:

is_single() returns true if any single post is being displayed
is_singular() returns true when any page, attachment, or single post is being displayed.

But when I try to link a CSS file only into a post, it's still included in attachment pages like http://example.com/?attachment=123456789. Here's the code I'm using:

<?php if ( is_single() ) { ?>
    <link rel="stylesheet" href="style.css" />
<?php } ?>

Answer 1

The Codex page for is_single should be updated, what you're seeing is correct behavior. According to the main Conditional Tags Codex page:

is_single()

When any single Post (or attachment, or custom Post Type) page is being displayed. (False for Pages)

You can exclude attachments by also checking ! is_attachment().

Also, you may want to move that code into an enqueue style instead.

EDIT-

if ( is_single() && ! is_attachment() ) :
    echo 'is a single post and not an attachment';
endif;

Answer 2

To specifically target single items of a particular post type use is_singular():

if( is_singular( 'post' ) ) {
    // Only execute this block on a single page for a post with type 'post'
}