These settings can be changed via Settings > System Defaults.
Click the "Files" tab at the top left.
Files
Visibility of newly uploaded files
When uploading new files, set who the file will be visible to by default.
Searchable by all staff: All staff will be able to search for the file
Only searchable by uploaded user and admins
File Categories
Each of the file types listed can automatically be uploaded to a specified file category.
For example, if you upload a word document, you can automatically make it go to a "Documents" category. Each of these categories are optional.