I'm using QBO free trial. I exported from QBD to QBO. But i have export problem. It didn't export QBD ATTACHED FILE to QBO. Only QBD the attached file did't export.

Thanks for reaching out here in the Community, @dndsales1736. Allow me to share some insights about this matter.

Some data from QuickBooks Desktop (QBDT) are excluded when you convert it to QuickBooks Online (QBO). One of them are your attachments.

You’ll have to create a copy of those attachments to transfer to transfer them from QBDT to QBO. Then manually attach them again to the transactions or your client’s information in QBO.

The “Attach” folder can be found in the same location as your QuickBooks Company file. For more details about this process, you can visit this article.

Once done, you can now start adding your attachments in your QBO account.

Here’s how:
 

1. Go to Sales menu and select Customer.
2. Locate and select the customer file and the file that is to be attached.
3. Click Edit at the upper-right.
4. From the Customer information window, click the Attachments tab.
5. Click the Attachments icon and then open the file to add.
   
   For visual reference, you can refer on the screenshot below.   

         6. Select Save.

To add attachments to transactions, these are the steps:
 

1. Click the +New button from the left menu.
2. Select Invoice as an example.
3. At the bottom of the Invoice page, select the Attachments icon.
    

Additionally, you might want to give this article a visit for more information about how to handle attachments in QuickBooks Online.

To wrap this up, I'll be leaving this article where you can check to learn more about the limitations on exporting QBDT to QBO.

Keep me posted on how it goes. You can get back to me anytime if you have further questions about this or any additional QuickBooks related concern. I'll be around to lend you my help. Keep safe!

@dndsales1736 

You have to migrate the attachment files manually to QBO. Another option, consider buying a 3rd party conversion service to migrate your data including attachments.