the problem here is that you are trying to figure out 2 things at the same time:
1) how to create a relational database for your needs
2) how to use data assist to manage that database.
and you are approaching the problem from the data assist angle first, that's kind of putting the cart before the horse. you need to figure out your database structure first, before you begin trying to create the pages with data assist. If you don't figure how the database is going to work, it's going to make figuring out data assist impossible.
so, you have cases and files for those cases. you will need to use 2 tables for this
cases:
caseID - primary key
casename
etc...
caseFiles
caseFileID - primary key
caseFileCaseID - relates to cases.caseID
caseFileFileName - gets the name of the uploaded file
your insert form should insert the case data to the case table, then in another insert behavior, it can insert the case ID using the session that is created from the first insert to the files table and use the Server File Name binding from the WA Uploaded Files binding group to insert the file name to the files table.