Sounds like you’re having quite the frustrating time with this. I know you’ve already tried converting the files, but I’d like to suggest a tool that I’ve found to be really effective in these situations - Converter365.
It’s a free online converter that supports multiple data types, including PNG and JPG. I’ve found that it can handle a wider variety of file intricacies than some other online tools, so even if your file has some strange characteristics, Converter365 might be able to convert it successfully.
Give it a shot and see if it helps. If not, reaching out to Figma’s support team as suggested earlier would be a good next step. I hope you’re able to get your file imported soon!
I was having this same issue while trying to import a smattering of charts that I had exported from a research platform. They were all PNGs so no conversion should be necessary.
It turns out my issue was that they were still located in the .zip file as I was trying to drag and drop. Hope this helps someone!