file upload field: schema.type=Text, schema.dataType=file isn't handled well

[az143]

as per issue #437 backbone forms does not (yet) have a dedicated editor for file upload fields.
using dataType=file is a workaround that works /mostly/ but not completely:

1. on initial render with a blank model everything is fine.
2. no "change" backbone events are triggered when the user browses and selects a file.
3. if the model happens to have a value already, then the current Text editor tries to set that as the value for the file input field, which fails: just about all modern browsers refuse this and flag it as an insecure operation.