Add list of attachment icons and update example

[marceloandrioni]

Hello, just a simple PR in the docs with the list of icons (and their aspect ratio) for attachments. I got the info here.

[jbarlow83]

Thanks. Apparently in PDF 2.0, the builtin attachment icons are gone and PDF generators must supply their own icons, so it would be better to have an example that embeds an icon since that is backward compatible. I'll have to look into this.

[codecov]

Codecov Report

Merging #336 (1e9a947) into master (ddac180) will not change coverage.
The diff coverage is n/a.

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##           master     #336   +/-   ##
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  Coverage   96.02%   96.02%           
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  Files          40       40           
  Lines        4054     4054           
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  Hits         3893     3893           
  Misses        161      161           

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[marceloandrioni]

Hello @jbarlow8. Yes, I only find out about the absence of default icons in pdf 2.0 after doing the PR. Apparently the icons needs to be included as an appearance stream (but I have no idea how to do this...). I opened the issue #337 regarding this subject and attached some example files.
I am no pdf expert but please let me know if I can help with anything (maybe testing).