Allow multiple files to be uploaded to one file upload component at once

## Context

**Feedback from one Beta user on uploading a number of files at once:**

> The ability to attach multiple files to one attachment element/question would be useful. For many use cases, we may not necessarily have a pre-determined number of attachments to request and ask clients to upload any “relevant” materials.

**Feedback from another Beta user on uploading a number of files at once:**

> I was wondering if there were any plans to allow the attachment feature to allow for more then one file to be selected?
> - We have a use case which could require a person to submit 10 + files in one shot.
> - From a user experience perspective, having to select each file one at a time is not ideal.
> - Would it be possible to enhance the attachment functionality to allow the user to select multiple files in one shot?

More details here: https://gcdigital.slack.com/archives/C08SR5HRFTK/p1757592859319159

## Support tickets

https://cds-snc.freshdesk.com/a/tickets/29504

## Related

#5944

Start by locating the attachment or file upload component in the TypeScript/NextJS application and review related issue #5944. The work is complete when one upload component lets users select and submit multiple files in a single interaction, including the existing attachment use case.

Written by the indexing model from the issue text.