VERIFIED SOLUTION i

Resolve issue where it is Impossible to open XLSX file with FileOpenDLG() function in MapBasic

Product Feature: Programming/Scripting
 

Issue

Running FileOpenDLG() function in MapInfo Mapbasic window brings up the File Open dialog but the "Files of type" list just shows "Excel (*.xls) and not also *.xlsx  

Mabasic commands:

Dim s_filename As String 
s_filename = FileOpenDlg("","","XLSX","Open Table") 

Dialog

Open Table

This command works the same in 12.5.2 (32bit) and 12.5.4 (64bit).
 

Cause

"File of Type" list is not consistent. File > Open command shows "Microsoft Excel (*.xls, *.xlsx)" in the list.
 

Resolution

UPDATED: September 7, 2017
You can open *.xlsx files and select them. So technically the dialog works properly. It's just the list entry which is causing the confusion. 

Running the fiollowing command in MapBasic window after  selecting a *.xlsx file shows, that the selection works:

print s_filename