Why some HEIC or HEIF files cannot be converted
A failed conversion does not always mean a file is corrupt. HEIF is a flexible container and a converter can support only a subset.
Similar extensions can hide important differences
HEIC commonly refers to HEVC-encoded HEIF still images, while HEIF can contain other encodings, multiple images, or features a browser decoder does not handle. Apple's HEIF overview describes that flexibility. An intact file can therefore be unsupported without being corrupt.
A controlled failure case
The release check retains sample-500kb.heic, which the
fixture source page
publishes as a generic 1920 × 1080, 8-bit sRGB CC0 file. It is used
only to verify the tool shows its clear unsupported error and
follows a non-retry, no-upload path. This controlled case does not
mean similar files fail, and it does not describe the file as
corrupt.
What to do next
Try the original on the device that created it, update trusted software, or ask the sender for a JPEG export. Reacquire the original rather than a preview or renamed attachment. On Windows, HEIF and HEVC support can matter, but it will not make every variant compatible.