It appears that the attachment control on the form that you are attempting to use is not the correct type for the attachment field.
Here's a start from MS:
Attach files and graphics to the records in your database Scroll down a ways to the section for forms.
The attachment datatype is actually a special field that creates a hidden table that links the records in the hidden table to the record in the data table. So you basically have to open a recordset on the value of the attachment field to use the information contained therein which is what the attachment control will do behind the scenes.
And the following may also prove helpful:
How to: Work With Attachments In DAO
Look at the code carefully, notice how they open the recordset on the field.
Personally I do not use the attachment field except for very special circumstances as the tend to fill up the available file space quickly and are a bit clunky to use in VBA or even Macro scripting for automation. Instead I tend to store the link to the file in a text field and pull from there when needed.
If you have a more specific question or can provide more detail about what you were expecting to happen and what is actually happening we can be of more help.