What I did check, apex_application.g_fnn array do not work for this.
From Google I did found that it has work in previous APEX version.
Something has changed and e.g. Denis example for multiple file browser do not work anymore.
I did manage upload files to wwv_flow_files table when using apex_application.g_xnn.
I did add derived column to report. To HTML expression I did place
<input type="file" name="x0#ROWNUM#" />
This way you could have report that shows max 20 rows in pagination page.
But I did not find way to identify file uploaded to flow files.
In After submit process apex_application.g_xnn seems have null value,
so I could not query wwv_flow_files by name column and insert file to my own table.
I think you looking for feature that is not included to current or older versions of APEX.
It is not bug or issue if previously not supported workarounds do not work any more in new version.
I see that this is new feature request.
You can create feature request from below link.
First try search if somebody have already request this. If there is request already, vote for it.