GetOpenFilename - FileName argument's syntax confusion (*.*)

No different other than the way the filter description appear in the dropdown list.

The convention is
Group Description (*.extension)

If you look at Excel's, or most other software, Open dialog the File types are described in that way.

The behavior of this string has been thought a bug by some but persists. As with all Microsoft code, I have been unable to find a rigorous specification for this string, only a broad description and simple examples.

Why is it like this? The answer is probably lost in some retired Microsoft engineer's brain.

My interpretation of the specification string is this BNF, although Microsoft never expresses it this way:

wildcard_list ::= wildcard[; wildcard]...
wildcard_list1 ::= wildcard_list
wildcard_list2 ::= wildcard_list
description ::= arbitrary text description for display
specification ::= description[ (wildcard_list1)], wildcard_list2

The weird thing is that wildcard_list1 and wildcard_list2 do not have to match. This invites the question as to why the specification is not simply
specification ::= description (wildcard_list)

Among the oddities:

If you omit the wildcard in parentheses, the wildcard after the comma is used for the filter and also inserted into the dropdown box item, as described by the OP.

FInfo = "Text Files, *.txt"
wildcard1.JPG

If you put a wildcard that does not match the wildcard after the comma, the one in parentheses will take precedence.

FInfo = "Text Files (*.pdf), *.txt"
wildcard2.JPG

FInfo = "Excel (*.xlsx; *.xlsm), *.txt"
wildcard3.JPG