don't explode when there's a dot in the path
don't explode when there's a dot in the path

closes FOO-1194
flag=none

If people manually construct paths trying
to target a specific file, instead of
a simple failure the react_files
endpoint would try to render a template
in the format of the file extension.

TEST PLAN:
  1) create a folder in a course
  2) visit a path UNDER that folder with
     a dot in the path (doesn't matter whether
     it exists or not) like
     /courses/3/files/folder/MyFolder/my_file.css
  3) you should be taken to the files area for the
     contextual course, not a page error. (404
     response from the API will occur in the network
     panel still for non-existing file)

https: //github.com/rails/rails/issues/28901#issuecomment-297747521
Change-Id: I7384af5bc0a684dc580907ae7f5f5f959387ad15
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/252507
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>
Product-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>

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