Consider context of image warnings

[sybrew]

In c73839e, I've added warning messages when selecting an inconsistently supported image extension.

This warning will also be shown when selecting an image for the sitemap logo or Local, which may not apply there.

Therefore, I already kept the warnings as generic as possible:

• Warned: "The file extension "%s" is not supported on all platforms, preventing your image from being displayed."
• Forbidden: "The file extension "%s" is not supported. Choose a different file."

'Warned' means that a social media platform doesn't support embedding the image. For example, Facebook or Discord don't support webp images.

'Forbidden' means that TSF filters the image from its output during sanitization. This process doesn't occur for all image display scenarios, creating the discrepancy and inaccuracy of the warnings.