If you won't use some unique string when building a key, you'll reach very soon the problem of a key override.
dsca007.jpg is not very unique. there is a big chance that a file with the same name will be uploaded to your s3 bucket. In this case you'll lost the old file or get an error, depends on your configuration.
e.g. this is what we are using:
https://s3.amazonaws.com/bucket_name/user_media/videos/screenshots/cmXRyLRQxe9R139023426817_vid001.jpeg
where the prefix: cmXRyLRQxe9R139023426817_ is a self generated string we build and concat to the original file name: vid001.jpg before uploading to s3.