Preserve plain API errors at document-upload UI boundaries

When an Axios client normalizes rejected responses into plain objects, a UI catch guarded only by instanceof Error discards actionable server messages. Document OCR can succeed before upload validation rejects the same file, leaving users with a misleading generic submission error. Use the existing API error reader at the display boundary and attach conversion guidance only to confirmed format-validation failures. Preserve size, authorization, timeout and storage messages separately. Re-exporting an image can resolve a format mismatch; renaming an extension cannot convert bytes. This does not establish the rejected file's exact encoding or justify expanding supported formats.

Applies when: document analysis and upload are separate stages using normalized API errors. Keywords: Axios upload OCR MIME format validation error message re-export