First of all, you need a reader node in your workflow, like a File Reader or a Table Reader. The node has to be already configured with a valid filename for the whole process to work.
After you have created and configured the reader node in the workflow, go to the file system and open the corresponding folder. The folder for the node should have a path like <WORKSPACE_FOLDER>/<WORKFLOW_GROUP>/<WORKFLOW>/File Reader (or Table Reader).
In the node folder create a new folder, name it "drop", and fill it with the file to be read. This triggers the creation of a flow variable containing the filepath at the next node reset.
Go back into the workflow editor of KNIME and reset the workflow.
Reopen the node for configuration, select the "Flow Variables" tab, open parameter DataURL for the File Reader node, and select the new flow variable "knime.node(drop). This flow variable contains the path to the file in the local drop folder. At this point the reader node is configured to automatically read the file in the drop folder. If you go back to tab "Settings" you will see the message "The DataURL parameter is controlled by a variable".
Now reset the workflow if you have executed it and export it with the option "Exclude data from export" disabled.
To export a KNIME workflow, right-click the workflow in the Workflow Projects panel and select "Export KNIME workflow".
If you now reimport the workflow that you have just exported, you should get the same workflow, with the drop folder in the reader node's folder, with the input file in the drop folder, and with the reader node automatically configured to read the data file in the input folder.
I found this trick extremely useful to export data and workflows at the same time without losing any configuration settings, especially if those workflows were then supposed to be used by others.