Windows 7 – How to Find All Files That Are NOT of a Specific Type/Extension in a Folder

Here’s a script (a.k.a. a “batch file”) that will do it for you in case you don’t want to download a program from somewhere:

@echo off
setlocal enabledelayedexpansion
for /r %%F in (*.m4a) do (
    set mp3name=%%~dpnF.mp3
    if exist !mp3name! (
        fc/b %%F !mp3name! > nul
        if errorlevel 1 (
            echo Warning: %%F and !mp3name! both exist but are not identical.
        ) else (
            echo DEL %%F
        )
    )
)

Interpretation:

• for %%letter in (wildcard) do some_command scans the current directory for files matching the wildcard and executes the command with the variable identified by the letter set to the filename.
• for /r %%letter in ... is the same but recursive.
• ( … ) lets you specify a block of commands instead of just one.
• As indicated above, %%F is the name of the M4A file that was found.
• %%~dpnF is the drive letter, path, and (base) name parts of %%F –– i.e., all but the extension.  To this we append an .mp3 extension; i.e., a filename that is the same as %%F except it has an .mp3 extension.  Assign this name to the mp3name variable.
• If this file exists, do the next six lines.  (Note that we are again using ( … ) to specify a block of commands.)
• fc/b %%F !mp3name! > nul –– fc is file compare.  fc likes to compare text files, so we say fc/b to do a binary comparison.  So fc/b %%F !mp3name! compares the two files.  > nul means don’t display the results of the comparison on the screen; we only want the script to know the result of the comparison.
• if errorlevel 1 means do the following command(s) only if the immediately preceding command “failed”.  In the case of fc, “failure” means that the files were not identical, so we have the script tell you that Hey_Jude.mp3 and Hey_Jude.m4a both exist but are not identical.
• Finally, DEL %%F means delete the file.  (echo DEL %%F means display the delete command, but do not execute it.)

Setup:

• In Windows Explorer, go to the root (top) of your music directory, do New Text file, and give it a name.
• Double-click on it to open (edit) it in Notepad. Copy and paste the above script.  Save and exit.
• Back in Windows Explorer, rename the Text (.txt) file to have a .bat extension.  Ignore the warning; click on “Yes”.
• Start a Command Prompt.  cd to the above directory.  Type the name of the script (batch) file (you may type the .bat extension, but you probably don’t need to).  Verify that the list of DEL commands it reports looks reasonable.  If it does,
• Go back in Windows Explorer, right-click on the .bat file, and select Edit.
• In Notepad, change echo DEL to DEL.  Save and exit.
• Back in Command Prompt, run the command again.

To remove all files with a specific extension can be done with something like:

cd C:\ path\to\directory\extension
del /s *.extension

For removing empty folders I would suggest looking at a question from stackoverflow:

https://stackoverflow.com/questions/7831286/how-to-delete-empty-folders-using-windows-command-prompt

EDIT: Obviously I suggest caution running this sort of command on an entire drive. There might be something you need. So do be careful.