These placeholders mean that Airtable is still working on generating your thumbnails. This process takes time.
If you see a paper clip icon, that means Airtable doesn’t support generating thumbnails for your attachment file type. Not all file types are supported. Make sure that yours are.
💡Tip
This extension doesn't support converting web pages to attachments, only files that are hosted in the cloud. You can try looking into this extension instead.