How do I retrieve the actual file modification date of an attachement

[Bill Bailey]

I have a user macro that generates a table of attachments, parsing information from the title, and displaying some of its properties.

I know I can access the attachment dates with:

$attachment.getCreatedDate()

$attachment.getLastModifiedDate()

But these dates are for when an attachment is uploaded to a page, not the date of the actual file.  How can I retrieve the modification of the source file?