SharePoint Designer created display forms do not display attachments when present.

To workaround this issue users can create a code snippet and insert the markup as needed via the code view.

Alternatively, users can just paste the markup that is use to create code snippets into the custom list form.

The following markup can be used to create the code snippet:

To create a code snippet in SharePoint Designer 2010 do the following:

1. Click the File menu inside of SPD 2010.
2. Then click Options and under the general tab pick Page Editor Options.
3. Switch to the Code Snippets tab.
4. Click the Add button and give it a name like attachmentFix.
5. Provide a description to help given context to the code snippet.
6. Paste the following markup to automate the task of adding in the attachment functionality for display forms.
7. Click OK and then OK.
8. Now, when working with a custom list form and it requires the above markup perform the following in code view: Control + Enter.
9. This will bring up the built in code snippets and the one created. Pick attachmentFix and all the markup will be inserted.

This can be further automated through a group policy or other delivery mechanisms. For example, Windows 7 uses the following path to store custom code snippets:

C:\Users\UserName\AppData\Roaming\Microsoft\SharePoint Designer\Snippets

The following document explains the code snippet's internals:

http://msdn.microsoft.com/en-us/library/aa218645(v=office.11).aspx#odc_fpcustomizingsnippets_tocreateacodesnippet

The file SnippetsCustom.xml contains the following after performing the steps above: