Search for "unknown" extensions
It is currently not possible to search for files with extensions that are not recorded in the Format Library, such as .md or .cxml, because they are not included in the autocomplete list. So there is no way to build sets of files with such “unknown” extensions. According to Ex Libris, there is an open issue on this subject: DPS-16975. I would like to suggest that increased attention be given to this issue and that a solution be planned for one of the upcoming releases.
Included in 6.0
We are pleased to say this will be part of the next Rosetta 6.0 release.
Yvonne Tunnat commented
I also like to add this would not only be nice for unknown extensions of "known" (known to the FL) formats, but also for formats, which are classified with fmt/unknown by the Rosetta FL.