In version 3.4 that was the case. To overcome that part, in my implementation the slash commands posts a link to my web interface that has the files (I was also dealing with attachments among other things)
It seems like version 3.6 might support attachments or some form of it now. I noticed that a slash command we had from our time with Slack is now bringing the extra data that we sent as attachments, which was not doing before.