When a Custom Audience is created in your Denim platform account, the .csv file that was uploaded was hashed locally on your browser. That means neither Denim, nor Facebook possesses the original file.
Curious what hashing is?
At its core, hashing is a way of converting pieces of data into a series of numbers and letters. The hashed data is sent to Facebook through your Denim platform account, and the hashed ID's are matched to Facebook users' hashed ID's. Matched records from your uploaded file to users on Facebook are built into the Custom Audience, and the unmatched records are ignored. Once matches are determined and the Audience is created, both the matched and unmatched hashes are deleted.
Ultimately, neither Denim nor Facebook ever has access to the data supplied during the process of creating a Custom Audience.