Hi!
Sorry for delay.
I checked your site and found, that when user tries to upload large file, it got 429 error from server (too many requests). This happens because uploader used chunked file upload method (each file divided to parts, each 1 Mb size, and for each of this part uploader send request to the server). Please try to contact your hosting provider and ask to increase requests limit.
About your second question – unfortunately, there is no option to make some similar to functionality, that was shown on Dribbble example.