As for the concrete case: have you tried doing that in parallel? In QML it was called a WorkerScript or so iirc. Besides, I don't think you have any other way to find out the duration than to query every file unless you use tracker or any other indexer - which you don't want. Maybe you could use some libmediainfo or so to query the data about the files and something for querying the MIME type (ignore the file if the MIME is bad). You could use a QThread so as to make it parallel. But I guess you can't do much more with QML only. You really should ask for details on the mailing list or IRC because I don't know which libraries are on SFOS and which not.
As for the image querying, I don't get what you mean.
Anyway, you could simply document the quirks and say: "it's simply all that I need" If someone needs more, they can always fork and implement it themselves and don't have to mess with the boring UI stuff.