Image component: pass allowedTypes to MediaPlaceholder

[benlk]

Description of the Change

This PR:

• allows the Image component to enforce the use of a specific file type
• updates docs to add instructions
• updates example code to use this parameter

Closes #239

How to test the Change

Use the sample code provided in the Image component README. Verify that it does not accept, via upload or picking from the library, a file which is not a .gif.

Changelog Entry

Added - Image component now supports specifying filetype via allowedTypes

Credits

Props @benlk

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[github-actions]

Size Change: +63 B (0%)

Total Size: 65.3 kB

Filename	Size	Change
dist/index.js	65.3 kB	+63 B (0%)

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[fabiankaegy]

This is a great addition :) Thanks!