As others have already written: when I read "Deluxe Paint", I automatically assumed that the artwork was done on an Amiga, because when you think Deluxe Paint, you think Amiga (I wasn't even aware that there was a PC version until now, or maybe I learned it at some point in the past and then forgot it again). And with that assumption, having no limitations on filenames or extensions actually made perfect sense.
And BTW, that ".lbm" extension should actually be ".ilbm" (interleaved bitmap), same as ".jpg" should actually be ".jpeg". The MS-DOS limitations live on long after MS-DOS is no more...
While all my friends used pc paintbrush, I had DP2e on the PC. It felt very limiting when I was on one of their computers. I have nearly zero artistic talent, but could still turn out some passable stuff thanks to things like the perspective tools. I hadn't heard the .brush version before and just assumed it was guybrush.bbm
Too late to edit my comment now, but obviously if you were going to use that test, you have to make sure to make it clear (without being too obvious) that you were talking about the MS-DOS version.
And BTW, that ".lbm" extension should actually be ".ilbm" (interleaved bitmap), same as ".jpg" should actually be ".jpeg". The MS-DOS limitations live on long after MS-DOS is no more...