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Issue Description:

Swift-DocC generally allows users to describe images with just the filename (ImageName), leaving out the file extension (.png).

For example, if a user included ImageName.png in their documentation catalog, they could refer to it with:

![Image description](ImageName)

However, when referencing images in Tutorials using the @image directive, DocC requires users to be explicit and use the file extension:

@Image(source: "ImageName.png", alt: "Image description")

This is confusing, so we should allow the following syntax:

@Image(source: "ImageName", alt: "Image description")