You can go to the extraction section by either clicking on the administration icon on your top right corner and clicking on "Add-ons", or by simply typing "g+g" and searching for "Extract Attachments". Once you are in the extraction section, you will see the below screen:
From here, you need to select your project(s), whether or not choose if you want to zip/unzip your extraction and a valid path on the server where JIRA is installed. Once you have filled the needed informations, you have to simply click on the "Extract" button. Now JAM will start extracting and collecting all the attachments of the selected projects. The extraction time depends on the size of the attachments and the server specs.
As you can see in the previous screenshot, that once the extraction is done, a new record is added in the history. You will see it's physical location on the server, name, the selected projects in that operation, the user who has triggered it and the size of the file. You will be able also to either download the file locally or simply delete it from the system.
You can also select the record and click on "Clear" button to remove the record from the history but keep it physically.
Also you will be able to download the extractions to your local machine or delete them from the server.