Check array fields normalizations when converting to multipart/form-data

[ganigeorgiev]

Related to pocketbase/pocketbase#4150.

[ganigeorgiev]

A partial fix was implemented in master.

The "fix" is partial because there are still 2 edge cases that are not handled - when a json field value is empty array (eg. []) or array of strings (eg. ["a","b"]).

The reason for this is because the SDK doesn't have information about the field types and doesn't know which field is a json or an arrayable select, file or relation, so it can't serialize it properly on its own as FormData string value.

If you are having troubles with persisting json values as part of a multipart/form-data request the easiest fix for now is to manually stringify the json field value:

await pb.collection("example").create({
  // having a Blob/File as object value will convert the request to multipart/form-data
  "someFileField": new Blob([123]),
  "someJsonField": JSON.stringify(["a","b","c"]),
})

A proper fix for this will be implemented with PocketBase v0.21.0 where we'll have support for a special @jsonPayload multipart body key, which will allow us to submit mixed multipart/form-data content (kindof similar to the multipart/mixed MIME).