You can try using File Listener data instance to read the file from a folder. In Service-File rule use File at a time processing method to process the file and assign the contents of the file to a text property something similar as below.
When you upload a file using the File control, file gets copied on the server and the path of the file is available on pxRequestor.pyFileUpload property. You could then use standard rule-utility-function (RUF) uploadFile to get the file content in base 64 encoded form in a Map. Check uploadFile RUF for more details.