(July 28, 2017, 5:19 pm)ProfessorCrastinator I saved the gifs from someone's Tumblr account, but uploading the files to stash does let me put the gifs in to the post as images rather than attachments. Problem solved, I guess!

Sorry I can't be of more help. The script appears to be rejecting the images and I would guess this is related to the number of frames. There are no settings related to GIFs except file size limits, however there's probably an upper-limit on frames per file somewhere inside of the attachment script (even if the size limit is large enough to accommodate 50 frames.)

I'll assess this later in the evening to see if anything can be found.