You can hook up a modem or even just a serial link and transfer files.
Or, just take the disk drive out, put it in your "regular computer" and copy the files to your "regular" hard disk.
If it's a hard drive, that probably won't work. Hard drives now are IDE, a drive from 1981 is probably some obscure technology like RLL (there were 3 or 4 as I recall).
If it's a floppy in the ATT, there still could be a problem. The connectors back in the day were sometimes 'card edge' style, like the connector edge of a PCI card.