Perform CSV checks we do for Entity uploads for CSV form attachment uploads

Users sometimes upload badly-formatted CSVs as form attachments. In particular, they sometimes have rows with commas in values not escaped with quotes. Currently, they have to test their form to identify the issue. Central already performs some CSV validity checks for Entity uploads. Those would be valuable for form attachments as well.

Inspect the existing CSV validation used for Entity uploads, then trace the form attachment upload path to find where equivalent checks belong. Reuse the relevant checks for attachment uploads and verify that rows with unescaped commas are rejected before users need to test the form.

Written by the indexing model from the issue text.