[Dangeranger]

Yes, this approach can work, unless the system which Rails/mimemagic is deployed to has restricted network settings. Under a restricted network setting, I believe the only solution which will work is to use a different package offering similar behavior, under a non-GPL license, or to re-implement the existing behavior using the freedesktop.org.xml as an input/output specification, rather than a source for derived code.

[nitrogen]

Presumably one could download the xml as part of the gem installation process (e.g. using mkmf as if it were a C extension, but there are probably simpler ways), so doing a local install at container build time would store the data with the gems in the Docker image (or local bundle if not using Docker).

But a mime database seems an awful lot like an uncopyrightable list of facts.

[Dangeranger]

Downloading and using the XML file as the source for some transformation would produce what is considered a derivative work, which would thereby be covered under the GPL. By accessing the file contents through an "abstract interface" this limitation could be avoided.

As I understand it, because the XML file contains instructions with regard to how and where to read files for the purpose of discovering their MIME type, those instructions are copyrightable. I could be wrong, as I am not a lawyer.