If this is new behavior, it is likely you have a virus. First try saving, and instead of letting the computer do it, type that last little bit yourself, if its a picture .jpg word documents as .doc, etc. Save it to the desktop just to be able to see it faster. If your file-name looks like "filename.jpg.jpg" then the program is not having a problem appending to the end of the filename, so the problem is likely going to be in the registry. (File associations, etc) Sometimes your error occurs when "saving as" with a file type of "any" and no specified file type in the name. Other times it occurs when a virus corrupts the file associations. Use Malwarebytes Anti-malware to scan, be sure to update it first, this will clear any virus files. (At least most of the major ones.) It can be downloaded free at: http://www.malwarebytes.org
Then download and run the temporary file cleaner and registry correction tool CCleaner, from http://www.piriform.com/ccleaner
Be sure to update both, and foliow directions after scan. IN CCleaner, you will want to run the temp file cleaner (which is the default start page) and the registry repair tool (registry tab), run the registry tool until there are no more errors. (analyze, and if error, run.)
McAfee, while relatively common, is not going to catch everything, and Geek Tech Tool Box, may be workable, but I cannot be sure of its abilities. Reasons I recommended what I did rather than to say "scan with what you have on-hand" (In my experience, McAfee is able to be bypassed by a good number of virus and malware.)
The fact you can see thumbnails is a good thing, it means something is corrupted in the file linkages (probably going to be registry or the program you are trying to save the files from.
For file association fixes:
As for the extensions themselves, make sure the "append file extensions" check box is set properly in the preferences for CS5. If you want to type it yourself, then not worry about changing it. Just remember you will want to type that pesky *.xxx at the end when you save (whatever the file type is)