Fix for attachments in Svelte useChat

[tyfiero]

Made a fix for attachments to work with Svelte. Took me longer than I'd like to admit to find out how to fix this. Works well for us

[cs50victor]

🚀. this was a very tricky bug to figure out. @lgrammel @MaxLeiter

[lgrammel]

cc @jeremyphilemon - was this already implemented or would the solution be more involved (with file lists etc)? I don't want Svelte to have deviate from what we implemented in React

[tyfiero]

cc @jeremyphilemon - was this already implemented or would the solution be more involved (with file lists etc)? I don't want Svelte to have deviate from what we implemented in React

Well this does work with multiple files in experimental_attachments we've tested it, its essentially the same code from the react package, I just added the two lines that were missing in svelte.

[lgrammel]

@tyfiero the changes for attachment support in React were more complex:

• Add experimental support for managing attachments to useChat #2226
• fix (ui/react): allow sending empty messages with attachments #2732
• feat (react): support sending attachments using append #2976

is all of this implemented for Svelte?

[tyfiero]

We opted to port our app over to react from svelte. It's clear the svelte package doesn't have, and will never have, the same support that the react package will. I won't have the time to implement those other attachment features in svelte. This PR can be closed.

[MentalGear]

I think feature parity should be an overall goal, no matter the front end library - especially for svelte that is officially supported by vercel.