[lxsmnsyc]

Problem

A file or a blob only reaches a server function as its only argument. Anywhere else, the call fails before it's sent:

await upload(file, { tags: ["a"] });   // SerovalUnsupportedTypeError
await upload({ file });                // SerovalUnsupportedTypeError
await upload(formData, 1);             // SerovalUnsupportedTypeError

Calls with more than one argument, or with a file inside a value, go through seroval. Start's plugin list has no plugin for Blob or File.

Change

When the arguments hold files, the client sends multipart form data:

• The serialized arguments go in the args field. Each file goes in its own file<i> part, so it's sent as it is, without base64.
• The new BodyFormat.SerovalFormData ("8") marks the request.

The files are found by a seroval plugin created for the call (createUploadSerializer). It replaces each file or blob with the index of its part wherever seroval finds it: arguments, arrays, objects, Map, Set, and FormData entries.

• A file that appears twice is sent once and arrives as the same value.
• A blob from another frame is detected by its shape and copied, because FormData only takes its own realm's blobs.

On the server, extractBody reads the form and deserializes args with the usual chunk size limit and a plugin created for that request (createUploadDeserializer), which turns each index back into its part.

• A missing part, a bad index, or a missing args field is rejected.
• Other bodies are read without that plugin, so an index in a plain seroval body fails to deserialize instead of meaning anything.

Calls without files are sent exactly as before, and the single-argument fast paths (FormData, File, Blob, …) are unchanged. serializeToJSONString, serializeToJSONStream and deserializeJSONStream take per-call plugins, tried before the built-in and user plugins.

This follows the upload design of brumate's server functions. The difference is that a seroval plugin finds the files, instead of a walk over arrays and plain objects.

Limits

• The whole body is read with request.formData(), as the existing single-FormData path does. There is no total size limit.
• Files arrive as File values. A Blob comes back as a File named blob, with its type kept.
• GET server functions put their arguments in the URL, so they still can't carry files.

Note for later

uploads.ts has a comment comparing this with seroval's binary mode (in development on seroval's binary-mode branch):

• Binary mode would be better for the arguments in general. It's one byte format, sends raw buffer bytes, and validates its input.
• Form parts are still better for files. BlobPlugin and FilePlugin read the whole file with arrayBuffer() before sending, and sending binary output as it's produced needs a streamed request body (duplex: "half"), which not every browser supports. A File in FormData is read by the browser as it sends.

A migration can move the arguments to binary mode and keep files as parts. Servers should accept both formats for a while.

Verification

• 10 new unit tests in fns/uploads.spec.ts. They do a full client-to-extractBody round trip with: a file plus metadata, files in arrays/maps/sets, FormData as one of several arguments, a file used twice, a blob's type, a cross-realm blob, a call without files staying as text, and the three rejections.
• New server-function-upload route plus a Playwright test. It sends a file, an object holding blobs, and a FormData with a file in one call. The test fails without this change.
• packages/start: 276 tests, typecheck, build, format check.
• apps/tests: unit, Playwright default (45), bundled-dev (45) and base (3) all pass.

The 1.x version is #2360.

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