-Filter only accepts a single string. -Include accepts multiple values, but qualifies the -Path argument. The trick is to append \*
to the end of the path, and then use -Include to select multiple extensions. BTW, quoting strings is unnecessary in cmdlet arguments unless they contain spaces or shell special characters.
Get-ChildItem $originalPath\* -Include *.gif, *.jpg, *.xls*, *.doc*, *.pdf*, *.wav*, .ppt*
Note that this will work regardless of whether $originalPath ends in a backslash, because multiple consecutive backslashes are interpreted as a single path separator. For example, try:
Get-ChildItem C:\\\\\Windows