You uploaded a PNG file with a transparent layer to SharePoint. Everything went fine, except the thumbnail and preview of your image: everything that is supposed to be transparent is now black.
For every image file, SharePoint creates a thumbnail and a preview and saves them as JPG files. But JPG doesn’t recognize transparent pixels and so it replaces them with black pixels.
SharePoint saves thumbnails and previews in a hidden folder of your library, such as:
To find the exact URL:
SharePoint replaces the dot of the initial file extension with an underscore, and adds '.jpg’ at the end of the file name: ‘Approved.png’ becomes ‘Approved_png.jpg’.
If you cannot open a SharePoint folder in Windows Explorer, you can also upload the file using SharePoint. In that case, you will not see the file you have uploaded in the Sharepoint list because the folder is hidden.