Fix: banner support valid image types

[Ayontikapal]

Description

This PR adds file type validation for event banner uploads to ensure that only supported image formats can be selected and processed.

Fixes #5368

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have run local unit tests using npm test and verified they pass
• I have run npm run lint and resolved any new errors or warnings
• I have verified responsiveness and visual alignment on desktop and mobile viewports

[vercel]

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[Ayontikapal]

@SandeepVashishtha Please review this pr under gssoc'26