Turns a custom format's name into the extension its files get

All 2 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.

The name arrives without the dot, and the extension is the name with one. A format named html therefore writes .html files. A name that a built-in format already uses is a name like any other here. A separate rule says which templates that name finds.