We need to extract the file name from a custom field with type document. I get ”unsupported data type ’DOCUMENT’” when I try to query it. Any way around this?

If you want to access files stored in the file cabinet then use FILE instead of DOCUMENT. Ex-

SELECT id FROM FILE

Document type fields are currently not supported in suiteQL. Create a custom text field on whatever record you have the document type field, this field should source the internal id of the file from the document field. Then you can do a join and get the filename based on the internalid of the new custom field.