Most Amiga software reads the header to work out what a file is then falls back to the ext. Irfanview is one of the few Windows tools I use that also does this, and then offers to rename it properly.
Looking in an Opus file I see Ogg as the first 3 bytes, and Opus at 28 bytes in.
As long as this does not vary and the tag info correctly stored after, it can be used to infer what decoder is needed.