I know you posted this question a long time ago, and that episerver has moved on since, I still have the same issue that you mention above, and I have posted that question here referring to your original question. I'm hoping I/we will get a response, but I also thought that perhaps you have found a solution / reason..?
I've been running a few tests with the search function in the Filemanager and came up with the following:
search for a file called logo_man.jpg
Search: jpg - fails
Search: *.jpg - fails
Search: logo - fails
Search: logo* - succes
Search: logo*.jpg - succes
My question: Is it my misinterpretation of how search should work, that I fail to find anything on logo without the wildcard?
Also, is it correct that you don't find any jpegs when doing a search on jpg or *.jpg?
Looking forward to your answer,