I get the same error, it's strange, now I'm trying to copy my Odoo10 filestore folder to my Odoo15 server and then import the ir_attachment records without the binary data, the import works fine but the data field is empty, I don't know how to fix it

Hi,

Try to export these fields shown in the picture below,

In v15 you can directly import the attachments, Also consider that the extension is not in .css/.js/xml that record may get some importing issues, all other extension records can import in the normal method.
Also, try to import less than 50mb data others may get loading issues based on the data size

Hope it helps