It is not possible to upload a file to a BPMN task directly. To make sure that a file is linked, you simply transform the file into a url/link. You can also choose to add a piece of content to the specific object. As soon as you open the process-object in BPM, you will find several fields on the right where you can enter information.
To enrich a BPMN view, you can use the BlueDolphin standard "BPMN Object description" questionnaire. You can change the content of the questionnaire just like any other questionnaire within BlueDolphin. As soon as you click on an object on a BPMN view a field will appear on the right (If not, please use these buttons and/or ).
Now you see what information is available for this specific part of the process. You can edit this information, for example by adding an url that takes you to a specific document/file/website.