mdfind piped to automator?

I have an Automator app called convert2xlsx.app that takes file inputs and runs the Convert Format of Excel Files action to turn .xls files into .xlsx files. I can call it from the command line for one file like so:

automator -i "some file name.xls" ~/Desktop/convert2xlsx.app

and it works just fine.

I also have a command-line query like this:

mdfind -onlyin . 'kMDItemContentType == com.microsoft.excel.xls'

that finds all .xls files in the working directory and below.

I would really like to marry the two together but I can't seem to get it to work.

My first thought was this:

mdfind -onlyin . 'kMDItemContentType == com.microsoft.excel.xls' | xargs -J {} automator -i "{}" ~/Desktop/convert2xlsx.app

but that only converts the last file in the results returned by mdfind. I also tried:

mdfind -onlyin . 'kMDItemContentType == com.microsoft.excel.xls' -0 | xargs -0 -J {} automator -i "{}" ~/Desktop/convert2xlsx.app

with the same result.

I was able to piece it together in Automator, using an AppleScript action to get the directory of the front Finder window, a Shell Script action to run the mdfind in that directory and then the Convert Format of Excel Files on the result of the mdfind. So, problem solved. Yay! But...

I would really like to know where I was going wrong on the command line, as I'm making an effort to learn Bash scripting and use it more. Any advice?