Well, the code that generates that particular error is:

unless ($content =~ /(jpeg|gif|png)$/) {
   return "this doesn't look like a jpg or gif!" 
}
[download]

The "D:" says you are probably on Windows which means that your browser could certainly be replacing the letter case of what you type with the letter case of the name of the file.

And if you have a file whose name is all upper case, then Windows has an annoying habbit of displaying that file name in lower case or simply capitalized. So you might try renaming the file so that the first part of the name contains some capital letters (not counting the first letter).