What kind of documents/files can you upload to the “Upload Materials” section in the applicant portal? Would it be appropriate to submit a resume through there?
No, do it on your common app.
@CrisprCas9 If I already submitted my common app would they be able to see any additions made following its submission?
No, the common app is set after you submit
I’d like to bump this because I’m hoping maybe someone could tell me what the upload materials function could be used for 
You can use it to upload your art portfolio, music etc, and any other significant documents you would like to share with Penn. Do not bother them with frivolous things, or anything that is already included in your application.
Thank you so much! @f2000sa