Fix bug where upload progress was not shown in FormAttachmentPopups

[matthew-white]

Closes getodk/central#606. The issue is that the event passed to the onUploadProgress callback no longer has a lengthComputable property. I've filed an issue about that at axios/axios#6290. However, the event now has a convenient progress property that we can use instead.

What has been done to verify that this works as intended?

It's difficult to write a test for this change for reasons described at #127 (comment). However, I verified the change locally.

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