I was having the same problem, initially.
I tracked it down to the fact that the images I was uploading happened to have a .jpeg extension, but the resized images were created with a .jpg extension. The code in file_gallery_filename_end_diff() assumes that the original and the resized images have the same extension — and since mine didn’t, I ended up with unexpected contents for the <span class=”diff”> element.
In my case it contained “-300×199.jpg;#;#;jpeg”.
@bryan, what was the extension on the files you were uploading?
I am not sure how/where the resized images are created — is this something the file-gallery plugin does, or the wordpress core?
