Fix for custom thumbs capitalization issue

[thejake420]

Please add a line or two of standard code to permit Coppermine to recognize custom thumbs regardless of whether the gif/jpg extension is capitalized or not,

Example:
Custom thumbs work when named thumb_filename.gif, but not when named thumb_filename.GIF. In the second case, it shows the default "zip" picture rather than the custom thumb.

In other words, capitalization of the extension makes the custom thumb unrecognizable to Coppermine 1.4.4 (and probably to other versions as well.)


Jake