Upon doing which operations on a file, does a user own a file in Chatter?

Answer: See the explanation.

Explanation:
You own a file in Chatter when you:
a. Upload the file on the Files tab.
b. Attach the file to a feed in Chatter.
c. Upload (i.e. contribute) the file in Salesforce CRM Content.
You own the files you upload or contribute.