@uppy/webcam: `preferredVideoMimeType` doesn't work

[Murderlon]

Initial checklist

• [X] I understand this is a bug report and questions should be posted in the Community Forum
• [X] I searched issues and couldn’t find anything (or linked relevant results below)

Link to runnable example

No response

Steps to reproduce

1. use @uppy/dashboard and @uppy/webcam
2. set preferredVideoMimeType to video/mp4.
3. Go to Firefox

It does work in Safari. But I would suppose mp4 is so common that Firefox would support it too?

Expected behavior

Result is video/mp4

Actual behavior

Result is video/webm.