[TYPO3-mvc] Problems with Upload file

[Chiến Nguyễn]

Hi all,

I am facing with problem in upoad file

When my form contains upload

<f:form.upload property="file" name="img" />

After click on submit form, i get error *#1251730701: The value must be of
type "Tx_XXX_Domain_Model_Step", but was of type "NULL".*
But when i use textbox replace upload, i can store input values into datbase

I don't understand the problems, could you help me? Many thanks

Regards,
Nguyen