Prevent re-uploading an already uploaded file
What is the problem you are facing?
When uploading a large amount of file it's sometimes hard to remember where you stopped before. And it becomes harder when upload fails for a few files in hundreds. So I often re-upload many files that are already on the server which is an hassle with a slow/bad connection.
The solution I can imagine would be to keep the SHA1 of each files on a database. Then, when a user want to send a file the SHA1 is computed client side then compared to known SHA1 on the server. If the file already exists, the client saves his time and doesn't re-upload it.
What are the possible drawbacks or issues with the requested changes?
Maybe on a slow computer, computing the SHA1 sum can be slow on large files. But, i guess it's still largely less than the time to re-upload it even on a good connection.