When we use InfoPath form as a workflow form with an attachment control, on selecting a file says that “The selected files was not found”.This seems to be a small bug:) in the product.
Note: It's not advisable or supportable to customize out of box files (comes with the SharePoint installation). There is a possibility that these files be overwritten in a future service pack, thus losing your changes.
a. Locate the ASPX page that your workflow form opens up in the _layouts directory on your server.
e.g., %Program Files%Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS
b. In this aspx page, locate the reference to MasterPageFile. This will look something like
c. Now open the masterpagefile that you found above in notepad and locate the form tag.
d. Add the attribute enctype="multipart/form-data" to this tag and save.
e. iisreset and try your workflow with file attachment again.
Hope this works!!!