Previously vips just called Mat_Open() to test if a file was a Matlab
save file, but this is rather unreliable. For example, some JPEG files
can crash libmatio, and it can incorrectly think that at least some JP2
files are Matlab save files.
Instead, look for "MATLAB 5.0" at the start of the file. This is really too
specific, the first 116 bytes of a Matlab save file are freeform text,
but in practice all Matlab writers use the first few bytes to record the
file type.
See https://github.com/jcupitt/libvips/issues/385
ef6d8f5979