Filter attachment image attributes to inject alt attribute to Image

wp-image-alt-attr.php

<?php

if ( ! function_exists( 'rtcamp_add_img_alt_attribute' ) ) {
	/**
	 * Filter attachment image attributes to inject alt attribute.
	 *
	 * @param string[]     $attrs      Array of attribute values for the image markup, keyed by attribute name.
	 * @param WP_Post|null $attachment Image attachment post.
	 * @return string[] Filtered attributes.
	 */
	function rtcamp_add_img_alt_attribute( $attrs, $attachment = null ) {

		if ( empty( $attrs['alt'] ) ) {
			$img_title = trim( wp_strip_all_tags( $attachment->post_title ) );

			$attrs['alt']   = $img_title;
			$attrs['title'] = $img_title;
		}

		return $attrs;
	}

	add_filter( 'wp_get_attachment_image_attributes', 'rtcamp_add_img_alt_attribute', 10, 2 );
}