Case-insensitive fallback/redirect

Question

I'm using some default images to show users the type of document that is used.

e.g. this image to show there is a .png file

In the /osr/icons/mime/ folder I have images for a multitude of files, e.g. avi.png, html.png, jpg.png, ...

So I simply need to get the file extension for my current document, for example png and display /osr/icons/mime/png.png

In case there is a file-format I don't have a predefined image, I'm using a rewrite condition to automatically show a generic image.

RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteCond %{REQUEST_URI} /osr/icons/mime/(.*).(gif|jpg|jpeg|png)$
RewriteRule ^(.*) /osr/icons/mime/generic.png

This works fine, except if the extension of a document is uppercase, e.g. .PNG instead of .png. In this case my code tries to display /osr/icons/mime/PNG.png which does not exist, an therefore shows the generic image.

Is there a way to change the RewriteCond/Rule to fallback to the lowercase image?

I found How do I do a case-insensitive mod_rewrite redirect to the same URL (mod case)? which recommends RewriteMap insensitive tolower: but I don't exactly know if/how this would work for me. Using [NC] didn't work either.

I know of the following work-arounds, which I don't really want to use:

• Switching to case-insensitiv file handling for the folder
• Dublicating List item all files in the folder to uppercase names (would not take .Png into account)
• Rewriting the code that loads the image file to automatically use lowercase