Appends the given entry to the request. This can be a simple key/value pair of strings or file uploads.

For a simple key/value pair, leave contentType and filename as null.

For file uploads, please note that many servers require filename be given for a file upload and it may not allow you to put in a path. I suggest using std.path.baseName to strip off path information from a file you are loading.

The contentType is generally verified by servers for file uploads.