Protect against incorrectly named files

What does this MR do?

There was an incorrect assumption that content_type_whitelist would parse the file contents to determine mime type. Unfortunately this only occurs in CarrierWave 2 and not 1.3.1 as we are currently using.

Currently the mime type is determined by the file extension despite the method name of content_type_whitelist. This change will add the CarrierWave 2 behaviour of content_type_whitelist.

This MR leans on changes made in MR !25401 (merged) which is currently under review.

/closes https://gitlab.com/gitlab-org/gitlab/issues/208265

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team