A typical file upload field with Ext style. Direct editing of the text field cannot be done in a consistent, cross-browser way, so it is always read-only in this implementation.
You can also render the file input as a button without the text field, with access to the field's value via the standard TextField interface or by handling the FileSelected event (as in this example).
The FileUploadField can also be used in standard form layouts, with support for anchoring, validation (the field is required in this example), empty text, etc.