Improve validation message when a design extension is unsupported

What does this MR do?

Improves the filename validation error message for a Design.

Previously this error message was output by Rails as:

Filename Only these extensions are supported: png, jpg, jpeg, gif,
bmp, tiff, and ico.

This changes the message to be:

Filename has an unsupported extension. The extension must be png, jpg,
jpeg, gif, bmp, tiff, or ico

See !21615 (comment 261398477)

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