Turns out that assigning the params to the procedure calls `read` on the
images files. Calling `read` moves the seek offset to the end of the
file; which means that subsequent calls to `read` return an empty
string.
Fix the issue by calling `rewind` before attempting to `read` the file,
to ensure the seek offset is at the beginning of the file.
Fix#4958
ed5f2fa35f