Uploading attachments to specific objects in Quickbooks Online (QBO) including Invoices and Bills is a straight-forward process, but if you are using the connector API Docs to upload these attachments there is one important note to make. The /attachments endpoints for these objects are available towards the bottom of the API docs within this resource:

/{objectName}/{id}/attachments

The reason for this is to keep the API docs more in line with how QBO actually handles the upload of attachments to objects as a separate API and not as a function of the Transaction Resource APIs themselves. For example you can make POST calls with the uploaded files to these endpoints:

/invoices/{id}/attachments
/bills/{id}/attachments