Analysis
Opening the file in a hex editor showed the header:
00 10 56 65 0D 0A 1A 0A 00 00 00 0D 49 48 44 52
This did not match any standard file signature. However, further down the bytes 49 48 44 52 clearly spell IHDR, which is part of a PNG header.
That suggested the file was actually a PNG with a corrupted magic number.
Fix
The correct PNG header should begin with:
89 50 4E 47 0D 0A 1A 0A
So I replaced the first four bytes with 89 50 4E 47
After saving the file and giving it the .png extension, the image opened successfully.