Request: keep the content type in file() and append files for keys ending with [] #146

Split from #133, part of #103. Moved to 8.0 because it changes the request that is sent.

• file() should keep an explicitly set content type (e.g. from json() or contentType()) instead of always overwriting it with multipart/form-data.
• A key ending with [] (e.g. docs[]) should append files to a list. Today a literal docs[] key is stored and only the last file is kept.