Just an update.
I’ve been looking into this, I was able to reproduce the issues you’ve described (upload failing mid-transfer for a large file) and then the next day (on a better internet connection) the transfer completed successfully – all the way up to 500 MB.
I suspect the issue may be more to do with execution times/timing out. Just a guess though at this point.
I also researched the memory_limit setting and what influence it has on upload limits – according to this discussion the PHP documentation is incorrect, memory_limit does not have any impact on upload limits. Source: https://stackoverflow.com/questions/5106871/relationship-between-php-s-memory-limit-upload-max-filesize-and-post-max-filesi
I tested this and it appears to be correct.
In addition to all this, it turns out that the WordPress administration (wp-admin) has a pre-defined memory limit – 256 MB – so even when I configured my memory limit to 1024 MB, the front end would report 1024 but the wp-admin would report 256 MB. There is a filter to change this, but given the first point that it has no influence on upload limits there is no point.
So with that all in mind in the next update I’ll remove memory_limit from the displayed value – but that doesn’t resolve the upload issues with large files.
To test the theory that it’s to do with timing out I will enable the chunking (in the next update) at a relatively low value, probably 5 MB. This way the upload script is refreshed every 5 MB. I may also look into the auto-resume feature for when an upload fails.
I do have a question, what is your upload speed for where you were accessing the form?
The connection it failed from is about 1.2 mbps, the connection it worked on (up to 500 MB tested) was 20 mbps (note I’m looking for the actual speed, not the speed you pay for but probably don’t ever achieve).