upload_mimes filter doesn't work occasionally

[dpacmittal]

get_allowed_mime_types() in wp-includes/functions.php contains static variable $mimes. So if get_allowed_mime_types() is called before a filter is added, apply_filters() for 'upload_mimes' is not called, rendering the filter useless.

It happened on one of my websites where get_allowed_mime_types() was being called even before 'upload_mimes' filters were added from the theme's functions.php.

Best fix is to remove the static variable. Thoughts?

Same for wp_allowed_protocols(): ticket:18268:18.

Static variable also should not be used because it makes breaking core functionality very easy for a plugin developers. A developer may simply call the function get_allowed_mime_types() in his plugin, which might cause the static variable to initialize and set with default array.

Note, at this point no filters would've been added because plugins are loaded even before 'init' (​http://codex.wordpress.org/Plugin_API/Action_Reference) . So if the function get_allowed_mime_types() is called at this point, $mimes is simply set to default array without running any filters. When the filter is added at a later stage, it is simple ignored because $mimes is true in the following code:

function get_allowed_mime_types() {
	static $mimes = false;
	if ( !$mimes ) {
		$mimes = apply_filters( 'upload_mimes', array(
...

Performance hit after removing static should not be much because it is not a very frequently called function.

Backupwordpress plugin breaks 'upload_mimes' filter and the developer has no clue about it, although he has not called get_allowed_mime_types() directly. He calls sanitize_file_name() which calls this function:

​http://wordpress.org/support/topic/backupwordpress-clobbers-upload_mimes-filter?replies=2

Same issue here:
​http://wordpress.stackexchange.com/questions/44777/upload-mimes-filter-has-no-effect

In [21541]:

Introduce wp_get_mime_types() for fetching the complete list of mime types. Remove the static caching of the types so that filters other than the first filter work. Use wp_get_mime_types() in do_enclose(). fixes #21299 #21594