Orgmode: image extensions

[tbruckmaier]

Follow-up from #5454: Right now, pandoc does not recognize certain extensions in orgmode as images, and therefore converts them into a link. Specifically uppercase extensions like PNG do not work:

$ echo "[[file:test.png]]" | pandoc -f org -t html
<p><img src="test.png" /></p>

$ echo "[[file:test.PNG]]" | pandoc -f org -t html
<p><a href="test.PNG">file:test.PNG</a></p>

$ pandoc --version
pandoc 2.9.2
Compiled with pandoc-types 1.20, texmath 0.12.0.1, skylighting 0.8.3.2

Maybe n the first step, the recognition can work case-insensitive.

And in the future it would be nice to have the extension list configurable. Cheers!