When uploading a large file the status will reach 100% and then stay there indefinitely. If you close the upload window the file will not appear in the folder, or if it appears the MD5 does not match.
After the server receives the file it then processes a few items such as the filesize and MD5 then updates the database with that info. Your browser is then updated with a new status "done". Normally this only takes a few seconds after the file is completely uploaded. If more than a minute or two has gone by at 100% it's probably safe to assume something has gone wrong. Usually when this happens we see this on inconsistent connections such as cell data and sometimes on wifi.
Just try again a little later. It's the internet. At some point or another this is bound to happen to everyone, especially on inconsistent connections. It's why we recommend that you verify MD5 on every file you upload.
If this *consistently* happens to you, you may want to consider using FTP Import. FTP transfers are resumable so if you do get a disconnect you can restart from where you left off at some later point in time. To get FTP Import simply use the Contact Page to send a request for FTP Access.
We are currently working on a UI update which will hopefully give a better notice than just being stuck at 100%. Thanks for your patience.