Comment 38 for bug 209509

Here's how I propose fixing it:
Change nsHTMLInputElement::IsHTMLFocusable(PRBool *aIsFocusable, PRInt32 *aTabIndex)
so that it returns *aTabIndex = -1 and PR_FALSE when mType == NS_FORM_INPUT_TEXT and the node is anonymous, and the binding parent content is <input type="file">:
http://mxr.mozilla.org/seamonkey/source/content/html/content/src/nsHTMLInputElement.cpp#2904

Then change onclick for the same situation so that it forwards the click to the file input's button.