Slack comes with limited file storage, so you eventually run out of space. Deleting files isn’t easy though: there is no way to bulk delete them using the web interface.
Slack forces you to delete one by one.
How to delete multiple files from Slack
In order to free up space, you’ll need;
Open your terminal and install
slack-delete-files package globally using Node Package Manager:
$ npm install -g slack-delete-files
You’ll be asked for your Slack token and whether you want to delete all files or only files older than 30 days.
Your space is probably back!
After deleting, you can uninstall the package:
$ npm uninstall -g slack-delete-files