Not sure why the attachment lost the extension. And actually the whole filename was completely different then my local copy. That's weird. It did give me an idea that turned out to work, but I couldn't begin to reason why. I have the file saved in google drive, and am using GNOME's online accounts to access google drive within my file browser. So when I went to the profile settings in Piefed to upload the file as my profile pic, I selected it directly from the mounted google drive in the firefox file chooser. I did that for both the profile pic and my banner image. They're in the same folder in the same google drive. The banner image worked but not the profile pic. So I just copied the profile pic to my actual local drive, and uploaded it to Piefed from there, and that worked.
So the issue seems to have ultimately been something about how firefox receives the file from Google Drive in GNOME's file system APIs, but it is inconsistent.
Anyway, problem solved.
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