I am making a QuickLook plugin that will be included with my application. It will be used to preview a file type that other applications also have QuickLook plugins for. Every time I attempt to test with my plugin (I put it into ~/Library/QuickLook/) it previews with another app's plugin. I am using qlmanage to preview. Is there a way to force it to use mine, at least for testing?
Generators embedded in applications are always preferred to plug-ins in ~/Library/QuickLook (which are preferred to the ones in /Library/QuickLook, which are preferred to the ones in /System/Library/QuickLook).
There is no way to change this priority. Also if 2 generators at the same level claims the same type of file, Quick Look will prefer one over the other, whichever you can't control.
At least for your test, you can force qlmanage to use the plug-in you want by using the -g option (but you will also have to specify the content-type with -c).
Try qlmanage -h to get all the options for your tests.