You can upload a text file by entering your text into the File Content field. Specify a filename including the file extension you want to use (txt, doc, csv). You will also need to enter a JIRA Ticket ID or Key corresponding to the issue you are uploading the attachment for.
You can download a text file by entering the Attachment URI. The "downloaded" attachment can then be used in following steps by selecting the data pill from the Download attachment step output.
Attaching the Attachment URI
In this example, the Attachment is being logged as a message to output to show how a downloaded attachment is read. Here is the logged output: