CAUSE
This behavior is by design of Microsoft Office and the Microsoft Office
programs. When you type a file extension in the File Name box in the Open
dialog box, all files in the current directory that have this extension
are displayed in the Open dialog box. Additionally, if the extension that
you type matches a single extension in the Files Of Type list, this entry
is displayed in the Files Of Type list.
However, the Files Of Type list displays the selected file extension only
if the entry is a single file extension. For example, if you type "*.xls"
in the File Name box in Microsoft Excel, the Files Of Type list displays
the entry "Worksheets (*.xls)."
This behavior prevents the list of files from displaying files with
extensions other than the extension you entered. For example, if the
Files Of Type list changes to Microsoft Excel Files (*.xl*;*.xls;*.xla;)
when you enter "*.xla" in the File Name box, then the list of files
displayed would include all those files with extensions matching *.xls,
*.xl*, *.xla, instead of only those matching *.xla.
Additionally, this behavior means that if the Microsoft Excel Files
(*.xl*;*.xls;*.xla;) option is selected in the Files Of Type list, and
you type "*.xla" in the File Name box, the list of files will include all
files that end in the following extensions:
*.xla
*.xla.xls
*.xla.xla
Note that because files with extensions similar to the last two
extensions in the list above are very unlikely to occur, the files
displayed in the Open dialog box should be the files that you were
looking for.