i also think now naming by it .xml its easier to view/edit it in other tools.
Whilst settling on a .xml extension does have the convenience of allowing the file to be opened in existing applications right away - it does also negate any uniqueness to the type, and also the future possibility of opening it in any specific application which may be dedicated to that type of object.
At the same time tho, it is equally easy to associate (for example) .jmexml with your favourite XML editor if that is the driving requirement?
This may also be setting a precedence for all other files related to JME - basically if the files structure is XML then the file extension would also need to be XML, regardless of the files actual contents?
I'd say I prefer .xml, since it's an xml file. Namespace should do the rest, and it is an XML file in the end and the standard is to use an .xml extension.
I personally tend to use two extensions if I want to know what kind of file it is (.jme.xml) but that's up to each developer. Directories could serve the same purpose for that matter.
Regarding operating system that use the extensions to decide how to open a file… that's an OS issue or a tool issue.
Also I learnt I'm using the wrong extension for my jbin format (.jbin instead of .jme).