I thought about it being a local setup issue as well. I had the app installed in Weblogic container at first, then did the standalone quickstart jar and both giving me the same behavior. tried it in Chrome 36, IE9, Firefox 31.
access.log
All the GET's look fine and the /POST/PUT gets sent while attempting the upload.
0:0:0:0:0:0:0:1 - admin 29/Jul/2014:09:20:59 -0300 "POST /libs/fd/fm/content/manage.json?func=uploadFormsPreview&folderPath=/content/dam/formsanddocuments&isIE=false HTTP/1.1" 200 736 "http://localhost:4502/aem/forms.html/content/dam/formsanddocuments" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1985.125 Safari/537.36"
127.0.0.1 - - 29/Jul/2014:09:23:24 -0300 "PUT /libs/sling/topology/connector.cbf27b45-6b9d-4f54-81a2-532c116b53da.json HTTP/1.1" 200 165 "-" "Jakarta Commons-HttpClient/3.1"
request.log. similarly clean.
error.log
'browser dev tool console' - are you referring to the log file page? Or somewhere else?
.. no action whatsoever...