Add file extension validation

[annaCPR]

What's changed?

• only allow pdf and html file extensions for documents

Proposed version

Please select the option below that is most relevant from the list below. This
will be used to generate the next tag version name during auto-tagging.

• Skip auto-tagging
• Patch
• Minor version
• Major version

Visit the Semver website to understand the
difference between MAJOR, MINOR, and PATCH versions.

Notes:

• If none of these options are selected, auto-tagging will fail
• Where multiple options are selected, the most senior option ticked will be
  used -- e.g. Major > Minor > Patch
• If you are selecting the version in the list above using the textbox, make
  sure your selected option is marked [x] with no spaces in between the
  brackets and the x