Simplify test that attaches a file

What does this MR do?

This MR simplifies the end-to-end test that attaches a file in the comment of an issue.

There's an issue that occasionally happens when running this test, where there's an ambiguous match of elements (see the below issue)

#205408 (closed)

This is a known limitation, as described in the comments on lines 16-18 of the below file:

https://gitlab.com/gitlab-org/gitlab/-/blob/master/qa/qa/page/component/dropzone.rb#L16

But, in the end, we just want to assert that an image would be rendered on the page, meaning that an img tag element would have a file path on its src attribute, which can still be done with this simplification, where we are not using the attach_file method anymore.

Closes #205408 (closed).

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