WordPress at a glance
action is not described

post-plupload-upload-ui action-hook . WP 1.0

This action is documented in wp-admin/includes/media.php

Usage

add_action( 'post-plupload-upload-ui', 'action_function_name_5559' );
function action_function_name_5559(){
	// action...
}

Where the hook is called

wp_print_media_templates()
post-plupload-upload-ui
media_upload_form()
post-plupload-upload-ui
wp-includes/media-template.php 269
do_action( 'post-plupload-upload-ui' ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
wp-includes/media-template.php 273
do_action( 'post-plupload-upload-ui' ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
wp-admin/includes/media.php 2219
do_action( 'post-plupload-upload-ui' ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores

Where in WP core the hook is used WordPress

wp-includes/media-template.php 25
add_action( 'post-plupload-upload-ui', 'media_upload_flash_bypass' );
wp-includes/media-template.php 267
if ( 10 === remove_action( 'post-plupload-upload-ui', 'media_upload_flash_bypass' ) ) {
wp-includes/media-template.php 270
add_action( 'post-plupload-upload-ui', 'media_upload_flash_bypass' );