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Check out SWFUpload, which is essentially a javascript api to flash's absolutely superior file upload handling capabilities. Best thing out there until the browsers finally catch up.
From link:
- Upload multiple files at once by ctrl/shift-selecting in dialog
- Javascript callbacks on all events
- Get file information before upload starts
- Style upload elements with XHTML and css
- Display information while files are uploading using HTML
- No page reloads necessary
- Works on all platforms/browsers that has Flash support.
- Degrades gracefully to normal HTML upload form if Flash or javascript is unavailable
- Control filesize before upload starts
- Only display chosen filetypes in dialog
- Queue uploads, remove/add files before starting upload
----- iframe upload -----
To start, you want to have an iframe on your page. This is meant for server communication. You'll hide it later, but for now, keep it visible. Give that iframe a name attribute, like "uploader" or something.
Now, in your form, set the target to the iframe's name and the action to a script you have on the server that will accept a file upload (like a normal form with a file upload). Add a link inside that form with the text "Add File". Set that link to run a javascript function which will add a new input to the form. This can be done via the DOM, but I would recommend a javascript library like jquery.
Once the new file input is added to the form, set the blur event of that input to a javascript function that will submit the form and then check it periodically for output. Reading an iframe can be tricky, but it's possible.
Have y