How do I upload files here on fluther?

You can not.

You can upload them somewhere else then link to them

Why would you?

Should be in Meta.

You have to link to a webpage, includes “photobucket” type sites.

You can’t upload files or embed them. I suggest you use Flickr as a means to link to images. For videos use YouTube.

This help page will show you how to customize your links.

Dropbox.com is a well known one for files
For images, lots of people here use imgur.com
Google Drive is also free.

If you want to stay anonymous then I would make a new anonymous account with one of these