When uploading a single file that exceeds 16MB, an error message is shown before submitting the query. When uploading a number of smaller files, though, whose cumulative total exceeds 16MB, no initial error message is shown. Below is an example where I attached 19 images whose cumulative total is 16.1MB. I can submit the query and the Assistant loads endlessly, then showing an unrelated error message.
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Error Message

Solution: implement an error message that signals when cumulative uploaded totals exceed 16MB. This partly relates to a previous feature request I submitted here: Add error message for when character/token counts exceed Assistant limits