From the Eudora Help Documentation: Attaching a File to a Message

Any file can be attached to and sent with a Eudora message. Most of the time, an attached document functions like a "rider" to the email message and does not appear within the message text. Instead, the name of the document appears automatically in the Attached field of the message header.
To attach a file to an outgoing message, do the following:

1. From the Message menu, choose Attach File [to New Message]. The Attach File dialog box appears.

2. Locate the file you want, select it, and click the Open button to attach the document to the current message or to open a new message with the file attached. You can add as many attachments as you want to a message, one at a time.

You can also drag one or more files from either the Desktop or the File Browser window onto the message window to attach them.
To detach an attached document before the message is sent, do the following:

1. Select the document in the Attached field.

2. Press either the Backspace key or the Delete key.