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Shell scripts start with "#!", therefore the primary list of magic
numbers recognises these as text/plain. However, AMR files also start
with "#!", which causes them to be misidentified.

BUG=813059

Change-Id: Icabf35126053a0e65c4d3a45c663c9122eb4a7a9
Reviewed-on: https://chromium-review.googlesource.com/c/1348887Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Anand Mistry <amistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610748}