[18.7] Fix image resizing assertion logic for RTE

What does this MR do and why?

This backports !217708 (merged) to 18-7-stable-ee due to a failing job: https://gitlab.com/gitlab-org/gitlab/-/jobs/13413945629

Fixes image resizing on upload assertion in RTE by uploading large enough image to cause resize in new UI.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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