Enhancement: Optional input file type extension to hint at best loader for Buffer/Stream

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## Question about an existing feature

Is it possible to pass a hint to libvips to know which loader to use?

### What are you trying to achieve?

Using libvips (with libraw enabled) via Sharp to open RAW files. This works fine for most files I tested, but it fails for NEF files, which have a TIFF header. When opening a NEF file in Sharp I get a very small TIFF image (thumbnail?), not the full image.

### When you searched for similar issues, what did you find that might be related?

I suspect this could be because of the TIFF header of NEF files. I think libvips tries [to detect NEF files based on the filename](https://github.com/libvips/libvips/issues/3380), if sharp hands off a buffer or the already opened file maybe that's the reason why the file is then loaded as TIFF.

Start by tracing Sharp's Buffer/Stream input path and how it delegates loader detection to libvips, using the NEF/TIFF behavior described in the issue as the reproduction case. Check the referenced libvips issue for loader-detection constraints. Done should include an optional extension hint that selects the intended loader and coverage for NEF input delivered through a Buffer or Stream.

Written by the indexing model from the issue text.