S3视频上传,文件不转码

I've recently pushed a video uploader for this website I've been working on for months. Recently, we have had two separate instances where the video the user uploaded wasn't completely uploaded resulting in a broken file that wont play. Currently, we are not sure as to why something like is occurring, but we have a couple of hunches, namely the files that are being uploaded are too big to handle and the user's upload speed is horrible. I was wondering what I can do to handle situations such as these where the upload conditions are less than optimal.

I upload firstly to the tmp directory on my Linux server then move the file to S3, our system is for end users to upload and we limit files to 40GB.