How to add "alt" attribute for image during upload at WP front-end?

Question

At WP Front-end I create a upload form for users.

That form has three input fields (title, tags and upload).

So I want to add "Title - Tag - file_name" as alt attribute for images during the file upload.

How can I do with my code below?

if ($gui == 'upload') {
    if (!function_exists('wp_generate_attachment_metadata')){
        require_once(ABSPATH . "wp-admin" . '/includes/image.php');
        require_once(ABSPATH . "wp-admin" . '/includes/file.php');
        require_once(ABSPATH . "wp-admin" . '/includes/media.php');
    }
    if ($_FILES) {
       foreach ($_FILES as $file => $array) {
           if ($_FILES[$file]['error'] !== UPLOAD_ERR_OK) {
              return "Upload Err: " . $_FILES[$file]['error'];
           }
           $attach_id = media_handle_upload( $file, $newPost );
       }                        
   }
   if ($attach_id > 0){
       //and if you want to set that image as Post  then use:
       update_post_meta($newPost,'Uploaded_Image_ID',$attach_id);
   }
}

Answer 1

The alt text is stored in the postmeta database table by WordPress, therefore we have to update the postmeta of the given attachment. The reference is done via the unique attachment ID, and the meta_key we are targeting is the _wp_attachment_image_alt.

So, to set the alt text, we can do the following:

update_post_meta($attach_id, '_wp_attachment_image_alt', 'Your alt text');

https://developer.wordpress.org/reference/functions/update_post_meta/