Add type filtering in appearance page of the admin panel

What does this MR do?

Add type filtering in appearance page of the admin panel

Prior to this commit, any file type was accepted for header logo, fav_icon and logo in appearance page in admin panel. This commit enforces type filtering for these fields.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done