Setting a file object by using a variable with .Files - vbscript

Question

I'm using this with HP Operations Manager, which uses the PARAMETER section so you can define variables from within the HPOM policy. This currently works for the one file hardcoded, but I want to be able to use the PARAMETER to set a filename, so the script is universal.
What this does is to check whether a specific file exists, and sets a variable (Rule.Status = True) if it is older than some amount of minutes specified in the FileAge variable.
Right now I am using:
Set MonitorFile = MonitorFolder.Files("EDI.001")
That works fine. But when I try to do:
Set MonitorFile = MonitorFolder.Files(FileName)
It fails with following error:
Invalid procedure call or argument.
Am I doing something wrong? Is there a better way of using a variable in this scenario?
Here is the whole script:
'PARAMETERS START
'PARAMETER FolderName STRING DEFAULT "D:\RFInput\InBoxPO" VALUE "D:\RFInput\InBoxPO\" SESSION
Dim FolderName
FolderName = "D:\RFInput\InBoxPO\"
Session("FolderName") = FolderName
'PARAMETER FileAge INT DEFAULT "60" VALUE "1" SESSION
Dim FileAge
FileAge = 1
Session("FileAge") = FileAge
'PARAMETER FolderDisplayName STRING DEFAULT "InBoxPO" VALUE "InBoxPO" SESSION
Dim FolderDisplayName
FolderDisplayName = "InBoxPO"
Session("FolderDisplayName") = FolderDisplayName
'PARAMETER FileName STRING DEFAULT "EDI.001" VALUE "EDI.001" SESSION
Dim FileName
FileName = "EDI.001"
Session("FileName") = FileName
'PARAMETERS END
Dim fs, MonitorFolder, MonitorFile, objShell, MinutesOld
Dim objFile, listNames
' Set constants for working with files
Const ForReading = 1, ForWriting = 2, ForAppending = 8
Set objShell = CreateObject("Shell.Application")
Set fs = CreateObject("Scripting.FileSystemObject")
Set MonitorFolder = fs.GetFolder(FolderName)
Rule.Status = False
For Each objFile In MonitorFolder.Files
listNames = objFile.Name
If InStr(listNames, FileName) = 1 Then
Set MonitorFile = MonitorFolder.Files("EDI.001")
MinutesOld = DateDiff("n", MonitorFile.DateLastModified, Now)
If MinutesOld > FileAge Then
'Turn on for debugging - Wscript.Echo FileName & " is older than " & FileAge & " minutes in folder " & FolderName & "."
Rule.Status = True
End If
End If
Next
Set objShell = Nothing
Set fs = Nothing
Set MonitorFolder = Nothing
'END OF SCRIPT

Answer 1

Indeed access to specific items in the Files collection seems to work only with string literals, not sure why that is.
You can simplify the For Each loop, though:
For Each objFile In MonitorFolder.Files
If LCase(objFile.name) = LCase(FileName) Then
Set MonitorFile = objFile
...
End If
Next
If you require lookup by filename you could build a dictionary like this:
Set filenames = CreateObject("Scripting.Dictionary")
For Each objFile In MonitorFolder.Files
filenames.Add objFile.Name, objFile
Next
That will allow you to access the files by name like this:
Set MonitorFile = filenames(FileName)