fix(FileUploader): resolve component audit issues

[emyarod]

Closes #7448

This PR updates the file uploader examples to resolve component audit bugs. A few questions about the audit points:

Alert icon needs a black inner fill for dark themes.

for the black fill within alert icons on dark themes, we run into the same issue with notifications from before (#3216) except we're not able to force file uploaders to be on dark backgrounds. alternatively we can just apply the black fill across the board on all themes if that's fine

The container needs to be disabled-02.

which container is this referring to?

Testing / Reviewing

Confirm the file uploaders match the updated spec

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✔️ Deploy preview for carbon-components-react ready!
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🔨 Explore the source changes: 1a954fe

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😎 Browse the preview: https://deploy-preview-7474--carbon-components-react.netlify.app

[tw15egan]

LGTM 👍 ✅

[laurenmrice]

Default file uploader

• Header needs to say “Upload files”
• Button text needs to say “Add file"

Uploaded File
Invalid state

• There is still a shift down when the invalid state is applied. The background of the field should increase in height to accommodate the error message below it. But the text and icons in the field should stay in the same place and not be jumping down.
  

• There needs to be a 8px space between the alert icon and the close icon.
  

File Uploader story

• The length of the file changes in width depending on the length of the text inside of it. Can we fix this so the file always has a width of 288px like it does in the other examples we show?

[laurenmrice]

The invalid state looks perfect for the default size file! It is still jumping down for the field and small sizes.

[laurenmrice]

Looks great! 👏