I was asked by one of my colleagues to delete the the files of one inactive user since he's currently the top user by file storage. I know I can delete some of his files in the documents tab but is that all that contributes to the file storage?
As per Salesforce docs:
File storage includes files in attachments, Files home, Salesforce CRM Content, Chatter files (including user photos), the Documents tab, the custom File field on Knowledge articles, and Site.com assets
So you might need to update All records where the old user is owner so you can update related Attachment owner as well. Except these under File tab, Chatter file (if any) you need to check them as well.