Actually, it should be possible to get the necessary information with HttpComponents HttpClient wire logging in trunk. But in 1.0.1 wire logging won't work - you have to use WireShark there.
Looking at the 1.0.1 code, the way the info is transmitted is as follows:
String value = "Content-Disposition: form-data";
if (name != null)
value += "; name=\""+name+"\"";
if (fileName != null)
value += "; filename=\""+fileName+"\"";
value += "\r\n";
byte tmp = value.getBytes("UTF-8");
rval += tmp.length;
tmp = ("Content-Type: "+contentType+"\r\n\r\n").getBytes("ASCII");
... which means that there are two headers in the multipart form section of the document:
Content-Disposition: form-data; name=<name>; filename=<filename>
... where, for the content, <name> is "myfile", and <filename> is the file name.
If this is not what the multipart form poster in 1.1.1 is actually doing, I should be able to fix it to do what we need. But I'd like first to understand what it's currently doing before I start changing things, because if it is already working this way then the problem is that Solr changed too.