I think you just need two selection lists and two backup jobs: one for mon-thurs, and one for all files on friday.
To make the selection list for mon-thurs, start out by making the same selections as the friday job. Then click the Advanced button.:
Select the same path, but insert *.mpg for the file and click the exclude radio button.
Repeat for all the file types you want excluded.
When you are done, click on the Text view format and you should see a series of lines that read
\*.* /SUBDIR
\*.MPG /SUBDIR /EXCLUDE
\*AVI /SUBDIR /EXCLUDE
etc, etc.
Create a mon- thurs job using this selection list. Use the test run feature to see how much less data you will be backing up.