It is a known issue that the attachment feature in custom form web part cannot be used. Currently, there are two workarounds for you:
Note: You can use this workaround if attachment functionality is required for New or Edit custom list forms.
Use normal List Forms instead of Custom List Forms.
Note: You can use this workaround if attachment functionality is required for Display custom list form.
- Open SharePoint Designer 2007. Go to File | Open Site and connect to SharePoint site.
- Expand out Lists -> [List Name] -> open DispForm.aspx
- Highlight normal list form by clicking on it. This will select the entire webpart.
- Right-click on selected web part, and choose Web Part Properties…
- Expand [+] Layout, check option for Hidden, click OK.
- With web part still highlighted, hit the right-arrow key once. Hit Enter. This creates some whitespace to insert.
- Go to File | Insert SharePoint Controls | Custom List Form…
- Select appropriate list or library, content type, and type of form to create. Click OK.
- Put cursor in the “Expires” table cell, right-click | Insert | Row below. There should now be a new table row with two column cells.
- Put cursor in the left table cell of the new row, type “Attachments”.
- Put cursor in the right table cell of the new row, go to Code view, and paste the following code:
<SharePoint:AttachmentsField ControlMode=”Display” FieldName=”Attachments” runat=”server” Visible=”true”/>
- Save page.