Before uploading a theme or a plugin to Filezilla you will need to extract it. It is the extracted file that is going to be uploaded.

You have to do this via your PC - do not go into the top left pane to extract it. Once done go back to Filazilla.

Firstly, you need to find the folder you want to upload to in the top right pane. Open httpdocs not httpsdocs.

This will open more folders, locate wp-content.

Now locate your theme folder or if you're uploading a plugin then locate the plugin folder.

The folder you clicked on will display all your themes or plugins you've previously installed in the bottom right pane.

The last part

Now move over to your top left pane to find your extracted folder. The folder that you want to move will have lots of files and other folders inside which should display in the bottom left pane. Do not move those individual files, but rather the whole folder.

Once found, drag the folder across to the top right theme or plugin folder. The transition will now begin, which can be seen in the very bottom pane.