fix file warnings for specs on linux

when running the test suite in docker, there are several errors present
of the form `file: invalid option -- 'r'`. This patchset addresses the
problem by using the expaned option names which appears to work on both
platforms.

Test Plan:
 - specs pass Jenkins
 - on docker, specs in assignment_spec.rb do not throw
   `file: invalid option --'r'` errors nor fail with
   `unknown mime type unknown/unknown`

Change-Id: Id260c886920180bb17a8b8d6c88c9400906b93e0
Reviewed-on: https://gerrit.instructure.com/163204
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Derek Bender <djbender@instructure.com>
QA-Review: Derek Bender <djbender@instructure.com>

gems/canvas_mimetype_fu/lib/canvas_mimetype_fu/mimetype_fu.rb

@@ -22,7 +22,7 @@ class File
     if file.class == File || file.class == Tempfile
       unless RUBY_PLATFORM.include? 'mswin32'
         # INSTRUCTURE: changed to IO.popen to avoid shell injection attacks when paths include user defined content
-        mime = IO.popen(['file', '--mime', '-br', '--', file.path], &:read).strip
+        mime = IO.popen(['file', '--mime', '--brief', '--raw', '--', file.path], &:read).strip
       else
         mime = extensions[File.extname(file.path).gsub('.','').downcase] rescue nil
       end
@@ -34,7 +34,7 @@ class File
       temp.close
       # INSTRUCTURE: changed to IO.popen to be sane and consistent. This one shouldn't be able to contain a user
       # specified path, but that's no reason to not do things the right way.
-      mime = IO.popen(['file', '--mime', '-br', '--', temp.path], &:read).strip
+      mime = IO.popen(['file', '--mime', '--brief', '--raw', '--', temp.path], &:read).strip
       mime = mime.gsub(/^.*: */,"")
       mime = mime.gsub(/;.*$/,"")
       mime = mime.gsub(/,.*$/,"")