If the designer has a custom object called MyThing{}, the file MyThing.qml must start with an uppercase letter (as indicated in http://doc.qt.nokia.com/4.7-snapshot/qml-extending-types.html#defining-new-components)

This information should be more visible, particularly as on Windows the file case does not matter - that is, a designer can create a complete UI on Windows only to find that it does not work on Linux due to mismatched case. Furthermore, it should be noted that filesystem rules apply to the naming.

i.e.
MyThing.qml
On Windows - Mything {}, MyThInG {}, all work as fs is case insensitive
On Linux - MyThing {} will work, Mything {} will not