Convert all images to PNG on file upload

(Updated answer)

The sight() function not only uploads the image, but also creates an attachment (i.e. a post of the attachment type); hence, I updated the code — it’s now in a function named attachment_to_png, and that it uses the get_attached_file() to get the image file attached to that attachment, and the update_attached_file() to update the file path.

So please follow these steps:

1. Add this to the theme’s functions.php file:

   function attachment_to_png( $att_id ) {
       $file = get_attached_file( $att_id );
       if ( ! $file ) {
           return false;
       }
   
       // Check MIME and make sure it's not already a PNG image.
       $mime = strtolower( mime_content_type( $file ) );
       if ( ! preg_match( '#^image/([a-z]{3,})$#', $mime, $m ) ||
           'png' === $m[1]
       ) {
           return false;
       }
   
       $im = imagecreatefromstring( file_get_contents( $file ) );
       if ( false !== $im ) {
           $filename = pathinfo( $file, PATHINFO_FILENAME );
           $to = dirname( $file ) . "https://wordpress.stackexchange.com/" . $filename . '.png';
   
           // Creates the image and saves it in `$to`.
           imagepng( $im, $to );
   
           // Frees the image from memory.
           imagedestroy( $im );
   
           // Deletes the original file.
           unlink( $file );
   
           // Update the attached file.
           update_attached_file( $att_id, $to );
       }
   }
   

2. Add this after the remove_filter('intermediate_image_sizes_advanced', 'no_image_resizing');:

   // Convert the image to PNG and delete the old image.
   attachment_to_png( $newvidPix );
   

Additional Note

I don’t think this is needed.. so you should just remove it…

$mfile =  wp_handle_upload($files, $upload_overrides );

---

UPDATED Dec 03 2018 UTC

(Updated for the single file upload)

1. You shouldn’t use the $im stuff and simply use the attachment_to_png() function. And call the function like so: attachment_to_png( 123 ) where 123 is the attachment ID.

2. Use media_handle_upload() which creates an attachment post for the uploaded image, and not wp_handle_upload().

So try this:

if ( isset( $_POST["ebc_submit"] ) ) {
    $uploadedfile2 = $_FILES['ebc_upload'];
    if ( ! empty( $uploadedfile2['name'] ) ) {
        $upload_overrides = array(
            'test_form' => false
        );

        add_filter( 'upload_dir', 'wpse_141088_upload_dir' );
        add_filter( 'intermediate_image_sizes_advanced', 'no_image_resizing' );

        // Load media_handle_upload() and other media/image/file functions.
        require_once ABSPATH . 'wp-admin/includes/media.php';
        require_once ABSPATH . 'wp-admin/includes/image.php';
        require_once ABSPATH . 'wp-admin/includes/file.php';

        // $v_Id is the ID of the post where the image should be attached to.
        $endpic = media_handle_upload( 'ebc_upload', $v_Id, array(), $upload_overrides );

        remove_filter( 'upload_dir', 'wpse_141088_upload_dir' );
        remove_filter( 'intermediate_image_sizes_advanced', 'no_image_resizing' );

        // Convert the image to PNG and delete the old image.
        if ( $endpic && ! is_wp_error( $endpic ) ) {
            attachment_to_png( $endpic );
            //echo 'Success!';
        }
    }
}

UPDATED Dec 05 2018 UTC

If you’re 100% sure about not going to be using unique file names; i.e. when you upload an image with the same name as a previously uploaded image, then you can create a callback function which will be used by wp_unique_filename(), where you return the original (but sanitized) file name, like so:

// Add this to the theme's functions.php
function no_unique_filename( $dir, $name, $ext ) {
    return $name;
}

And then, in the $upload_overrides (see previous update), add unique_filename_callback like so:

$upload_overrides = array(
    'test_form'                => false,
    'unique_filename_callback' => 'no_unique_filename',
);