Dropbox, the popular online file-synchronization service, is offering up to 4.5GB of additional free storage for those who agree to test out a new automatic photo and video uploading feature, the company said in a forum post on Thursday.

The new feature, which is part of the new Experimental Dropbox build 1.3.12 software, enables photos and videos to be automatically uploaded from cameras, smartphones, and SD cards. Whenever a user plugs one of the devices into their computer, the software will automatically begin uploading the photos to their Dropbox.

"For every 500MB of photos and videos automatically uploaded, you'll receive another 500MB space bonus, up to 4.5GB total," Dropbox wrote. "To clarify, you can get up to 5GB of free space with this feature, if you count the initial 500MB space for the first photo import."

Dropbox 1.3.12 is available for Windows and Mac OS X users. It is not, however, available on Mac 10.4 and Linux. Windows users will need to have Autoplay enabled for the feature to work.

Users should make a backup of their Dropbox before upgrading to the experimental build, Dropbox warned.

"We'll need a few weeks to be certain of its stability," the company said. "We will rapidly release new builds as necessary to snipe the most common issues as they arise."

The premise behind Dropbox is it gives users access to their files on any computer or device they have at hand. The service stores files with strong encryption on multiple servers and lets users access their files from virtually any Internet-enabled device.

Dropbox received a PCMag Editors' Choice and four-out-of-five stars in an October review. With the service, users can receive 2GB of storage at no cost. Check out the slideshow below, as well as PCMag's review of the Dropbox iPad app.