Some clients may approach you with the backup file as .jpa archive(joomlapack) which cannot extract using the usual tools available in server. This can be done by using the third party script Akeeba Extract Wizard.
Download the Akeeba pack from here https://www.akeebabackup.com/download/official/akeeba-kickstart.html
Upload the kickstart zip archive again in public_html or in the same folder where the you have the .jpa archive. Extract the content of the archive. This will put multiple files in the same folder(you see many .ini files of it).
Access the kickstart.php script inside the unzipped kickstart folder via your domain and browser. For example:
(should load the correct path where your kickstart.php resides. If it is in a subfolder, load as http://yourdomain.com/subfolder/kickstart.php)
On the kickstart.php loaded on browser, you can browse the .jpa archive file exists in the same folder and go click Extract. You will see the extracted files and folders in the same folder where your kickstart package downloaded 🙂