I wrote a simple WordPress Plugin that upload all images inside a folder located on another server. The uploading process is working well, but for some reason when I go inside Library I have a blank thumbnail:

when I add the file using my own plugin, I also create the post inside the wp_posts table, in this case contains the following info:

and inside the wp_postmeta table I have the following:

is there any reason why the thumbnail isn’t showing up?
Kind regards.