Three options spring to mind:
- use Data Explorer to create a query for all Upload records with desired extensions and print the URL, then download them to a folder
- download an SQL backup, then do the same as above
- download a backup including all uploads (requires assistance enabling this temporarily on our hosting), then extract that backup
With the first two options you can also grab the original filename so you can preserve it upon download but there will be collisions.
With the third all the files will be named with their hash (the long hex string) but you can use the data from #1 or #2 to rename them.