Implement file upload and screen maintenance functions in SAP

I need the following to be implemented by a qualified SAP ABAP programmer who well experienced in development using ABAP Objects using global classes. The project consists of several milestones -= the description below is for the first milestone only. We have a program already available which contains 4 screens for 4 custom objects. The basic PBO/PAI are ready for these screens. Screens do not have to be developed. Part of these screens involve upload of structured XLSX or CSV files with all fields mandatory to populate the custom tables for the object in reference for each of these screens. Error handling details will be required and be provided to the chosen programmer. This is the first milestone.