StreamCopier.CopyStream does not copy the header of the file into the output

[gabriel-vasile]

After the discussion we had in gabriel-vasile/mimetype#310, I had a look at the copier.StreamCopier and I think there is a problem.

step1: extensionFromBody will read file header in order to detect extension. It reads from a bufio.Reader which, in turn, reads from resp.Body.
step2: Later, io.Copy(output, resp.Body) is called and, because the header was already read in step1, it is skipped.

You can confirm the problem by saving a URL twice:

• once using radiot_dumper (make sure extensionFromBody is called when you test)
• second time use the browser to save the file.
  Then, compare the file saved with radiot_dumper and the file saved using the browser. The radiot_dumper file will be smaller because it is missing the header.

[F0rzend]

Unfortunately, the problem has not been resolved. Apparently the point is that this is a stream, and not just a recording