Template:OBX mimetypes, used on Activity pages, adds pages to this category, and also to Category:mimetype_something. Some of those categories were manually made subcategories of this.
The only way to find all the Category:mimetype_something category assignments is to look through Special:Categories, e.g. http://wiki.laptop.org/index.php?title=Special:Categories&offset=Mimetype A semantic annotation like [[Handles mimetype::image/jpeg]] would be better.
The OBX templates reappear on translated activity pages and repeat the same English-language category assignments in them.
See also Data file formats
This category has the following 10 subcategories, out of 10 total.