When naming files for use on 50megs there are very few restrictions. You do need to avoid using spaces, since spaces are used to indicate where one command ends and the next begins. Remember that 50megs file names are case sensitive, so it is a good idea to keep all of your files in one case.
Lower case is recommended.
It's generally a good rule of thumb to begin a file name with a letter whenever possible, although it is not necessary. If you name a file using the wrong extension it will prevent your visitors from correctly open the files. It is also important not to remove the extension by renaming the file.