Help! E-receipts uploaded but could not assigned to a trip

On ECC 6.0 sp 4, In Travel Expense Manager, I was able to upload a PDF document to the Content Server. But when I selected the receipt and assigned it to the trip, I always got "Document type missing for e-receipt '****.pdf'; please contact administrator.

below is the message:

Document type missing for e-receipt '****.pdf'; please contact administrator

Message no. /OTTM/TM416

Diagnosis

You are trying to assign a document to this trip but no document type is customized for this mime type. The administrator has to customize the document type before you can complete the assignment.

Procedure

Please contact your administrator to check the document type customizing.

Where do I need to change the document type? This has taken me several days.

Thanks in advance!