Construct MIME form post in static method

Moves MIME form post construction from Request.submit to
static method Request._build_mime_request

Request param 'file' has been a special case invoking MIME
handling.  Site.upload has been updated so param 'file' is
a mime_param, and api.Request has no special case for it.

Change-Id: I09abe7e8350c034f1d5a2c81d99063ab3c3f7924