Add SVG icons for file types: doc, empty, jpg, pdf, and png

[sgimama]

• Added file-doc.svg for document representation.
• Added file-empty.svg for empty file representation.
• Added file-jpg.svg for JPEG image representation.
• Added file-pdf.svg for PDF document representation.
• Added file-png.svg for PNG image representation.

Please fill in this template

• Use a meaningful title for the pull request
• Optimise SVGs before you raise a pull request

Select one of these, and delete the others:

Adding an icon: this will be marked as a patch

• Give the icon a meaningful name, preferably namespaced eg,
  car-windscreen rather than just windscreen.
• Add svg to the icons/ folder, and raise the pull request

If moving, renaming or deleting an icon: this will be marked as a minor

• Give the icon a meaningful name, preferably namespaced eg,
  car-windscreen rather than just windscreen.
• Describe why the icon was renamed, moved or deleted.
• If you are renaming an icon, and also adding an icon with the same name
  as the old renamed file. Please raise both of those in a single pull
  request, and annotate the pull request as a breaking change.
• With an all caps "BREAKING CHANGE" at the bottom of the pull request,
  describe a migration path for this.

BREAKING CHANGE:
IconA is now IconB
IconC no longer exists, please use IconX instead

Note, you don't need to tell us that IconA no longer exists, because of the
rename.

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Big icon changes will be marked as a major, but will probably be done manually
in 1 pull request, and versioned accordingly.