To list files of specific type(s) on the disk or directory including all sub-directories
get-childitem c:\ -include *.tmp, *.temp, *.bak -recurse
get-childitem D:\Projects\abc*\ -include *.bak -recurse
The commands above will list on screen all files with extentions TMP, TEMP or BAK located on disk C:\ and all subdirectories or located on disk D:\ in directory PROJECTS and all subdirectories with name starting with ABC and their subdirectories.
To delete files of specific type(s) from the disk or directoty including all sub-directories add
| foreach ($_) {remove-item $_.fullname}