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.

Work-around:

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

MasterPageFile="~/_layouts/application.master"

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!!!