most likely the include file is being included using the virtual() function instead of the require_once() function.
The virtual() function has a bug where it will write a carat at the start of the file causing the headers already sent error.
In the Site Settings, on the local info tab, there is a setting for "Create Links Relative To". Choosing "Site" causes Dreamweaver to use the virtual function. Chose "Document" instead, and Dreamweaver will use the require once function and avoid the headers already sent.
another thing to check is that the forms encoding type gets set to multipart/form-data.
If you insert the file field by going to Insert -> Form -> File Field, Dreamweaver will do this for you automatically, if you are just changing the hidden fields type to file manually though, you will also need to manually change the encoding type. do this by electing the form tag, and then in the property inspector, change the enctype dropdown list.
For storing the uplaoded file name in the database, you will need to use the data assist insert record server behavior.
See the "How to apply Digital File Pro to DataAssist Insert and Update pages" tutorial on the DataAssist support page: