The example js is not minified so it is readable. See file-upload.js. FileUploadField.js is the custom extension used to create the file upload field.
Basic FileUpload
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.
Basic FileUpload (Button-only)
You can also render the file input as a button without the text field, with access to the field's value via the
standard Ext.form.TextField interface or by handling the fileselected event (as in this example).
Form 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. This example also demonstrates using the
buttonCfg option to provide a customized icon upload button.