Sorry I'm not sure what you mean by the last modified date.
The Extract will be refreshed when you tell it to, either manual or on a schedule basis. Having the modified date updated on your drive will not trigger an extract refresh.
To see where your extract is pointing to, right click on the data source and go to properties:
Based on my previous email, I will want tableau to extract the date as of 29Jun18 (my last modified date for my source).
What I mean :
Date : 1 Jun 2018
Data last modified date : 1 Jun 2018.
Expected date from tableau : 1 Jun 2018
Date : 2 Jun 2018.
Data last modified date : 1 Jun 2018 ( files was not updated)
Expected date from tableau : 1 Jun 2018. ( as per last modified date)
Please see this thread which asks a similar questions: how to extract a character from an external file?
Unfortunately I can't think of a way of how you could do this as the modification datestamp sits within windows and Tableau cannot access those details unless it's in a data source.
Maybe there's some additional scripting that could help you do this, but unfortunately I personally don't know how to do this, sorry!