Believe it or not I have to do this also.
I have legacy ASP code that uses ASPUpload that I'd like to get working with my Flash CS5 and FlashBuilder Burrito front-ends.
it's just multipart encoded.
So, if you first test it with ASPUpload and a simple form, trying just FILE1 first, and it works with ASP backend then tackle the FlashBuilder side with some debugging.
ASPUpload is still an extremely embedded component on tens of thousands if not more sites, and it was just updated, but I think the previous version just before this update, the one that's out there in code on so many sites, that is what Flash needs to work with.
ASPUpload is doing its part, as it follows all the multipart encoded RFC rules and has worked for years.
I'd love to join you in finding out what's going on here.
This is a long-standing issue.
I've done PHP also. But again, enhancing an interface with FlashBuilder4 or Burrito, to integrate with working legacy code makes it hard when something is not working on the Flash side (it appears anyway) as ASPUpload form posts are extremely easy.
It is NOT a FLEX question.
FlashBuilder Creates FORMS for Posting just like any other Design Tool.
The RFC, FILE1 field, has been the method of a Form submitting Multipart encoded data for a VERY LONG TIME.
As such, it belongs right here in the FlashBuilder Forum where FORMS are created.
You Insert a form field, one that is used in multipart/encoded forms, called FILE1, or if it's for multiple file "Browse..." buttons on a Form (the best way to test for FlashBuilder now) you add FILE1, FILE2, FILE3.
ALL BROWSERS have for 15 years or so supported the FILE1 multipart encoded Form Submission.
This works when creating a form in Dreamweaver CS5, this works when creating a Form in any application that represents a Browser Form submission in anyway.
So it needs to work in FlashBuilder4 or "Burrito" -- either way, it's not the type of post you re-route to the FLEX forum to never get answered because they wouldn't quite know how to answer it without starting from the beginning.
Not sure what is your requirement. Flash Player already has FileReference which can be used to upload files
There are various examples on now to do this (http://www.google.co.in/search?sourceid=chrome&ie=UTF-8&q=FileReferenceflexASP.NET )
If your requirement is for Flash Builder to provide some kind of User Interface to generate code for this or have some "upload file" component in design view, let us know or file an enhancement at bugs.adobe.com/flex