On mobile, you can find the attachments option in the + More menu on the bottom of your screen.

From there you can navigate through your device file and also your cloud services to select the files you need to attach to your post, you can upload photos, videos, PDF, spreadsheets and more.

On the web app, click on the post and click Attach files from the bottom action bar. You can browse your computer, connect to Dropbox or other storage services, or simply drag and drop your files onto the screen.

On your computer, you can select files to upload or drag and drop files from your local folders.