"file not sent" from Android to Macbook, whereas they're paired I'm trying to send a file from Android to Macbook via bluetooth. I pair them successfully and when I'm sending a file from Android, it just ends up saying "file not sent". In the Macbook does happen nothing at that time, no pop up window or something.

That's it.
In System Preferences > Sharing > Bluetooth Sharing, ensure that Folder for accepted items is set to something. If it isn't set, incoming file transfers are rejected automatically.