Creates a new file in the directory.
Data will either be supplied as a stream resource, or in certain cases
as a string. Keep in mind that you may have to support either.
After successful creation of the file, you may choose to return the ETag
of the new file here.
The returned ETag must be surrounded by double-quotes (The quotes should
be part of the actual string).
If you cannot accurately determine the ETag, you should not return it.
If you don't store the file exactly as-is (you're transforming it
somehow) you should also not return an ETag.
This means that if a subsequent GET to this new file does not exactly
return the same contents of what was submitted here, you are strongly
recommended to omit the ETag.