Uploaded .xlsx files always need to be repaired

I just had a report that an uploaded .xlsx form needed to be repaired when a customer tried to view it. I have just randomly checked the last few .xlsx files and they all show the same problem. Excel (Office 2013) says there is a problem with the file, then manages to repair it OK. I have checked the original files and they open with no problem, so it seems as if the upload/download process is implicated.
Is this a known problem, and is there a workaround ?
The files are uploaded using the Browse/Upload buttons from an invoice usually.

Hi @Alchemyst

I’ve just replicated this issue, and will ask a colleague to have a look. Please bear with us, and one of us will come back to you shortly

It may be due to the way the storage provider encodes the file. To be quite honest, I don’t know! Although the MIME type appears to be correct. Other than that Google hasn’t revealed anything else conclusive here. We’ll make some further enquiries but we may need to revisit this later.