#upload with an actual File object unexpectedly closes the file

[contentfree]

In the case where you use #upload with an open file handle (instead of a string path), the file is unexpectedly closed in #write_next_chunk(file) after reading the last chunk. The closing is important when a File has been opened (when given a path), but is detrimental to further usage of an open File.

For now the workaround is to check if the file is closed after the upload and reopen it.