Keyframe interpolation is always enabled, you would require special setup or code to disabled it.
Are you perhaps using single images (image sequences) at your attachments?
Or did you export your skeleton as Image or Video sequence instead of as Data (json or skel.bytes)?
acai I've found several posts mentioning holding shift or using code, but I couldn't quite understand them.
Holding shift? This makes no sense. In general it would help if you could share references to the postings that you don't understand, then we can explain what they were actually describing.