CmdPal: Allow any image format as icon for protocol bookmarks (#42145)

## Summary of the Pull Request

This change attempts to load any supported image format as a protocol
bookmark icon, instead of restricting it to PNG only. The original
implementation handled only PNG (which are common), but the manifest
also supports JPG/JPEG extensions.

Reference: [UWP manifest schema –
uap:VisualElements](https://learn.microsoft.com/en-us/uwp/schemas/appxpackage/uapmanifestschema/element-uap-visualelements)


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## PR Checklist

- [x] Closes: #42144 
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/ThumbnailHelper.cs

@@ -242,8 +242,8 @@ public static class ThumbnailHelper
             }
         }
 
-        // 2) Handle .png files returned by store apps
-        if (iconRef.EndsWith(".png", StringComparison.OrdinalIgnoreCase))
+        // 2) Handle image files from a store app
+        if (File.Exists(iconRef))
         {
             try
             {