fix unsupported .txt files showing in the ePub
fix unsupported .txt files showing in the ePub

fixes CNVS-25501

test plan:
- upload a pair of .txt files
- one should use an underscore for spacing
- one should use a space for spacing
- both should be replaced in the ePub with the unsupported message

Change-Id: I5b0d6b8550a40e934f2a0973154e8a968b664e2c
Reviewed-on: https://gerrit.instructure.com/68223
Reviewed-by: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>
Tested-by: Jenkins
QA-Review: Rajkumar Kethavath <943d80ed9dc95d434849fae8b8fb2a95a056374d@instructure.com>
Product-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>

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