The following worked on my old installation of Outlook 2016 so it must be either a small typo or there is something wrong with the references.
The code is in Outlook 2016:
Sub Sample()
Dim myFile As Variant
Dim i As Integer
'Open File to search
myFile = Application.GetOpenFilename(MultiSelect:=True)
If IsArray(myFile) Then '<~~ If user selects multiple file
For i = LBound(myFile) To UBound(myFile)
MsgBox myFile(i)
Next i
Else '<~~ If user selects single file
MsgBox myFile
End If
End Sub
I get:
for this line:
myFile = Application.GetOpenFilename(MultiSelect:=True)
Here are my selected references: