Proposal: Automatic reading policy for unified attachments

[usnavy13]

With unified uploads a user attaches a file once, but LibreChat still decides what happens to it from the file type alone. A spreadsheet is text-extracted into the prompt at upload even when Run Code is enabled (#16618). A document the provider rejects for size is filtered out of the turn before File Search or Run Code could take it, and the model is never told how an attachment was read. The broader request is #10941.

Proposed scope:

• An opt-in llmDeliveryPolicy: automatic under fileConfig, settable globally, on endpoints.default, or for a provider or custom endpoint. The default classic keeps today's routing.
• The policy engages only on a turn where Run Code or File Search is loaded, from the agent builder or the message bar. A conversation with no file tool keeps classic routing and limits unchanged, truncated text included.
• On those turns each attachment's reader comes from its file category, the current provider's existing validators and limits, and the tools actually loaded. Spreadsheets (xlsx, xls, ods, csv, tsv, parquet) go to Run Code with no upload-time extraction. Other documents go to the provider within its limits, then to complete extracted text when it fits, then File Search, then Run Code. A document too large to send directly tries File Search and Run Code before text.
• Explicit destinations, legacyFileUploadUX and defaultLLMDeliveryPath still win.
• The model receives a compact per-turn list of how each attachment can be read and which it cannot. Text is derived from the retained original only when a reader needs it. A pending File Search file is indexed before the first search.
• No visual change, except that the file preview no longer shows a workbook kept for Run Code as raw ZIP bytes.

Affected areas: file reading types and the delivery-path resolver in packages/data-provider, a new packages/api/src/files/reading (upload decision, per-turn context, inventory), the agent upload and initialization paths in api/server/services, lazy provisioning for Run Code and File Search, and the fileConfig schema.

The implementation is open as draft #16848, based on upstream dev. I opened it before claiming the work; it stays a draft until this is decided.

Tracking issue: #17370. I would like to contribute this feature with agent assistance. Please approve this scope and assign the tracking issue to me.