Fix fallback for non-image files

[ThatKJ]

Fixes #63449

The image viewer was picking files based on their extension before checking if the file was actually an image. Because of that, a normal text file like test.pam would end up showing the broken image view instead of opening in the editor.

This changes the opening flow so that if the image viewer checks the file and doesn't recognize it as an image, Workspace can try the next opener instead.

Valid images still open normally, and image decoding errors are still handled by the image viewer.

Added a regression test using a text test.pam file.

Tested with:

• cargo test -p image_viewer --lib
• cargo test -p workspace --lib register_project_item_tests
• cargo fmt -- --check
• ./script/clippy -p project -p workspace -p image_viewer

Release Notes:

• Fixed non-image files with image extensions opening in the image viewer instead of the editor.

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[ThatKJ]

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	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[SomeoneToIgnore]

Thank you, this is a nice idea but the fix has to be a bit more generic, something similar to #63160 , and, ideally, avoid extra error handling in this way, as it's quite error-prone.