This message means this according to the Knowledge base:
An "Encoded Data Checksum" is a value which is calculated as a file is attached to a document. The value is then saved with the attachment. When the file is detached, the same method is used to calculate another checksum. The two values are then compared. If they are different, the error "Encoded Data Checksum Mismatch" will occur. The checksum is used as a way to ensure that the complete file was sent and received. The Checksum Mismatch means the data is not the same as where it originated. This error message indicates the attachment has been corrupted in transit on the network. The specific corruption could have occurred due to a network problem. For example, if NetBIOS packets are being reassembled out of order on the receiving side of a transaction, this error will occur.
The user had the problem trying to detach a word document created in 2000. They were using a release 5 client. I tried detaching it with a Release 4 client and it worked ok.
I reattached it and it then worked in R5 and R4.
Another option is to try and save it using a browser.