Can't upload ico favicons

[SychO9]

Bug Report

Current Behavior
ico favicons cannot no longer be uploaded.

Steps to Reproduce

1. Go to Dashboard Apearence page.
2. Upload ico favicon.
3. Notice it errors out with a 500.

Environment

• Flarum version: 1.0.3 (but introduced in beta 15)

Possible Solution
An abstract UploadImage controller class was introduced in beta 15, the makeImage method is expected to return a Intervention\Image\Image object, which is not the case for ico files where we return the file stream, this is because the GD extension used by intervention, does not support ico files.

https://github.com/flarum/core/blob/a077ae9ca36a8ad0adc25042118ba2407feaa5fe/src/Api/Controller/UploadFaviconController.php#L29-L42

We could remove the explicit typehint on the makeImage method, but that would break extensions, so the best thing to do is to throw a validation error on ico images in 1.x (since there is no way to fix this without making breaking changes) and we can fix the issue for 2.0

Additional Context
Relevant PR: flarum/framework#2477