Safari downloads completed but corrupted

I've been trying to download large video files from my personal cloud but every time the videos are "completed" downloading on safari they are not openable. The downloads say for example 1.4gb of 1.4gb and the progress bar will be completely full but they sit in this state forever and as a result are not openable. I am not closing the tab where the download sources from I simply go about browsing on other tabs. It happens at random and usually occurs when downloading multiple files at once. It's extremely annoying because it requires me to download the same files multiple times and wait very long, I really could use some advice as this is a pain in the neck. Thanks in advance for any help.