glocalfileinfo: Increase size of the content-type sniff buffer to 16KiB

It was previously 4KiB, but this isn’t enough for sniffing the content-type of some GPT disk images (they use 4KiB sectors, and the magic bytes are in the second sector). A buffer of 8KiB would work, but 16KiB seems harmless and more future proof.

Most of the time, the buffer size should be set by xdgmime anyway, based on the largest sniff buffer its database needs. Currently (with shared-mime-info master) that’s 18730 bytes, so even with a 16KiB buffer we’re going to potentially mis-identify a few file types.

Tested manually by modifying the example GPT image from shared-mime-info (https://gitlab.freedesktop.org/xdg/shared-mime-info/-/blob/master/tests/mime-detection/disk.gpt) to remove the magic at offset 0x200 and add it instead at offset 0x1000, then running:

cp shared-mime-info.git/tests/mime-detection/disk.gpt ./disk-without-extension
gio info -a standard::content-type ./disk-without-extension

It should print application/vnd.efi.img rather than application/octet-stream.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Fixes: #3186 (closed)

Closes #3186 (closed)