What it does is write a test.php file to that folder.
Not sure about this version but in prior versions that test file was required for it to work.
But when the paths are correct it will write the test file to the folder.
This is still correct for the 7.6.x series
Testing video attachments here on UBBCentral:
(I had to dig really far back in time to find a small mp4 video file.)
