OK, here is a way of doing this. There must be a shorter way, but can't think of one right now.

(DT_DATE) (left((DT_STR, 8, 1252) @[User::DateKey],4) +...

Please read this:
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/9f9812c3-56cf-4a76-ad48-fad7b03ad9ac/how-to-pass-filename-as-a-dynamic-variable-to-flat-file-source-in-ssis-?forum=sqlintegrationservices
and confirm that you have set the flat file source dynamically.

This thread has taken a strange turn – see the series of posts after Ed's post. I really don't see the issue.

This is simple in SSIS, and had you posted the question in the Integration Services forum, you would have had an answer by now.
Use something like (untested)
[code...

SELECT
  item_name
,  category = (CASE
       WHEN item_name IN ('charger', 'cable') THEN
         'electricity'
       WHEN item_name IN ('pen', 'pencil') THEN
         'accessories'
       ELSE
         'other'
     ...

You have missed a step, I think.
Create a new string variable, to hold the file name.
Configure the FEL to map the 'current' file name to the variable you have created.
Use...

Thom, I have not read your code in detail, but doesn't running a package execution in SYNCHRONOUS mode obviate the need for this polling?

Is G: a local or network drive? 
Does the SQL Agent user have rights to the folder?