1) Show only files with a special extension:
Use *.extension as filter. "*" replaces all filenames and ".extension" is to be replaces by your extension. For example if you want to show only mp3 files, use this as filter:
*.mp3
If you want to show only mp3- and wav-files, you can seperate both with a semicolon:
*.mp3;*.wav
More examples:
image filter: *.jpg;*.bmp;*.png;*.gif
audio filter: *.wav;*.mp3;*.wma;*.ogg
video filter: *.mpg;*.wmv
2) Show only files beginning with a special letter
That is what you have to use as filter: yourletter*.*
For example if you only want to see files beginning with an "a", use
a*.*
as filter. You can also combine with an extension, so using
a*.mp3
you will only see mp3-files beginning with "a". Again you can combine multiple elements with ";".
Tip: You can also use "?" instead of "*".
"*" replaces an undefined number of chars, "?" only replaces one letter. So if you only want to see all files with "a" as first letter and any second letter use
a?.*
Files like ab.mp3 will be displayed, however no files like abc.mp3.