Prerequisite
Also check SAP Notes:   SAP Note 2053696
URL Upload
<Host:Port>/sap/bc/webdynpro/sap/zrsplf_file_upload_v3?planning_sequence=<Planning Sequence>&sap-client=<Client>&sap-language=EN
Implement BAdI
ZRSPLF_FILE_UPLOAD
Class – Method that trigger BAdI
ZCL_RSPLF_FILE_UPLOAD – IF_RSPLFA_SRVTYPE_IMP_EXEC~EXECUTE
Step #6 trigger  l_r_badi->transform_file
Step #8 trigger  l_r_badi->transform_record
Step #9 trigger  l_r_badi->transform_data
My Case
Need to implement validation when uploading file before data goes into infocube.
What I’ve Done
Put validation logic inside method transform_file from BAdI implementation.
20170426-1107
Result
20170426-1111