Upload Import File

Upload a CSV or Excel file and get back its id and row count.

Nothing is imported by this call - the file is stored and parsed so it can be previewed and then imported. The 25 MB ceiling is the app's, enforced by reading one byte past it so an exact-boundary file is distinguishable from an oversized one even when Content-Length is missing or dishonest.

The row_count that comes back is what the import pre-check budgets against: it is treated as the upper bound on NEW contacts, so an import is refused if that many would breach the org's limit, even though some rows will resolve to updates.