In past for very large files, ive had luck using rillke's chunkedupload script setting the chunk size as high as it can go.

Several times I received an "Unknown error: "parsererror" message and the upload fails. The browser's developer tools' error console states:

This can happen if the API fails really hard and doesn't even produce output in the JSON format, or if you're behind a proxy that doesn't allow file uploads or otherwise doesn't let your request through.

Sometimes UploadWizard also just remains stuck, showing "Assembling..." in the preview box

Assembling could take a long time… How long have you waited? UploadWizard should give up with an error after 10 minutes.

In past for very large files, ive had luck using rillke's chunkedupload script setting the chunk size as high as it can go.

Hmm, UploadWizard always uses 5 MB chunks. Perhaps we should use bigger chunks for bigger files.

In T141229#2491015, @matmarex wrote:

Several times I received an "Unknown error: "parsererror" message and the upload fails. The browser's developer tools' error console states:

This can happen if the API fails really hard and doesn't even produce output in the JSON format, or if you're behind a proxy that doesn't allow file uploads or otherwise doesn't let your request through.

I am not behind a proxy.

Sometimes UploadWizard also just remains stuck, showing "Assembling..." in the preview box

Assembling could take a long time… How long have you waited? UploadWizard should give up with an error after 10 minutes.

I've waited about 12 hours.

So... is this actionable? If not, what data to provide and how?
Or does T130610 cover this already sufficiently?

No, T130610 was a different issue.

As for the actions, we could try to make the API fail in a better way, and make UploadWizard do something smarter when the API does fail like this. We could also try to split the file into chunks in a smarter way, depending on filesize.