Found this while testing MDLQA-1395 (A student can add images to a wiki). I uploaded an image file in the wiki Files tab. Then I went to edit a Creole-formatted wiki page, and the image file would not appear in the drop down for inserting an image. After some investigation, I found that the cause was that the file extension of the image file was in upper case. I then tried an image file with a lower-case file extension and that worked.

This looks very similar to MDL-30165 except that it affects a different area. The problem appears to be in mod/wiki/edit_form.php with this condition in the definition method:

if (preg_match('#'.$ext.'$#', $filename)) {

The fix for MDL-30165 fixed a similar issue in repository/lib.php by change the preg_match pattern to '#'.$ext.'$#i' (Note the addition of 'i'.)