From Xojo Documentation
The MIME name used to refer to the File Type in your code.
This is what is shown in some open/save file dialogs. The Name property can be specified by a constant. When using dynamic constants, the names are automatically localized on macOS. On other platforms, the Name property will return the dynamic constant value, which allows you to register/update your file type with the system using the localized name.
You can see a list of common MIME names by opening the File Type Group Editor and choosing the command bar button to add a common file type. The menu will show some common types and you can click "More.." to open a chooser at the bottom that shows many more.
The following defines the jpeg file type in code: