Refuses a format name that could not be an extension

All 3 scenarios hold

A project can generate a report in a format of its own, beside the three built-in ones. A pair of templates named after the format is the whole definition.

The name identifies the format, and every file of the report takes that name as its extension. The reporter therefore refuses a name that cannot be an extension. It refuses the name when the project declares the format, and not when it writes the first file.

Three names cannot become an extension: an absent name, a blank name, and a name that already starts with a dot. The reporter refuses each one as the project declares the format, and the message names what is wrong. The reporter writes no report under a name that cannot name its files.