rewrite url if file does not exist to a given subdir

Hi,

I like to obtain using .htaccess that requests in the format:

/usr/home/mine/public_html/files/(.*)/(.*)

where the first wildcard (.*) is a directory and the second (.*) is for
a filename.

will be rewritten if not found on the physical location to (where I
replaced the wildcards with there references):

/usr/home/mine/public_html/files/Image/$1/$2

but not if $1 is Image, because in that case it will if there is no
file, keep adding the Image subdir.

As an example:
request: http://www.mysite.com/files/nature/flower.gif
if found:
show the file on: http://www.mysite.com/files/nature/flower.gif
otherwise:
show the file on:
http://www.mysite.com/files/Image/nature/flower.gif

Thanks in advance for a solution,

Roderik