One can change the tabIndex of a file input in FF1.5 and verify the value has changed, but the browser will ignore the change and use whatever value was staticly defined at page load.

As a workaround, see if you can define ‘tabindex’ for all items in the markup. If you’ll be dynamicly adding non-file inputs, set their tabIndex to be the same as whatever they should come after in tab order — browsers use doc order to break ties in tabIndex values. To dynamicly add file inputs, put something like this in the markup:

  <input type=file id=file0 tabindex=N onchange… /><span id=file1container></span>

Then, when you add a new file input, assign to the innerHTML property of the span rather than using createElement. (You probably also want to style the old input as display:none, depending on what you’re trying to do.)  The tabindex of the new input can be N also.

UPDATE: Once the file input is filled, one might want to take it out of the tab order. Setting tabIndex = -1 has no effect in FF1.5, and setting the style to display:none (or visibility:hidden) will cause Safari 2.0.3 to ignore it when sending the request. I haven’t found a solution that works for all browsers.