You can upload a file with the file manager widget. You need to put it in a dataview that points to an entity that is a generatlization of system file document. Then you can pass that entity as an input param in your microflow and do anything with it you like.
However as Ankit pointed out, it really does sound like you would like to use the excel importer.
create a microflow that retrieves the template you are using from the excel importer package. check to see that the template is not empty. if it isnt, call the java action in excel importer called start import by template.
hope this helps.
the "basic" modeler created application only has a facility to import/export XML. However the APPstore has a component that you can download from the appstore and that will add functionality to your APP in the modeler.
Download the component from the APPstore and follow the guidelines to merge it into your project.
Now if you deploy you project you will find a specific set of screens that allow you to identify the EXCEL to import, define the parsing parameters, and actually import the files and process them into your application.
Note that this means that importing EXCEL stuff NEEDS to be done in a "life" environment.
Hope this clears things up for you