[2.x] Improve error messaging for unsupported file types in "appearance"

[claudiushenrichs]

Current Behavior

Uploading an SVG image shows a generic "Oops! Something went wrong." error toast.

Steps to Reproduce

1. Go to Administration -> Appearance
2. In the "Logo" section (Labelled "Upload an image to be displayed in place of the forum title."):
   2a. try to upload and SVG
   2b. try to upload a non image file like a PDF

Expected Behavior

The error message for unsupported (image) file types should state something more clearly like "The uploaded file is note supported. Upload images as WEBP, PNG, JPEG"

Screenshots

Environment

• Flarum version: 2.0.0-beta.7
• Website URL: https://nightly.flarum.site/admin#/appearance
• PHP version: 8.2.30

[esbeeb]

Dumb question: where does this image file format support come from, in all the underlying PHP/laravel/etc libraries? Maybe more image formats might actually work than just those 3.