Or to put it another way, don't upload more than you need to.
Some spreadsheets are huge consisting of thousands of rows and consequently thousands of calculations to work out the final nine boxes for the VAT100 return.
As spreadsheets get bigger and more complex, they take more time to upload, process and carry a higher risk of calculation errors.
There is also a problem with data protection. Many spreadsheets contain sensitive data, e.g. customer names, account numbers, supplier names etc. and it is not a good idea to share this information with any third party. This is true for data protection and business competition reasons. Your data is a valuable asset to your business and you should be very careful with how you handle it and with whom you share it. That includes mtdvatbridge.uk and HMRC.
HMRC's MTD VAT system only requires the nine boxes on the VAT100 form as well as the accounting period key, vat number and finalised acknowledgement. mtdvatbridge.uk only uploads the data to HMRC that HMRC require, no other user supplied data is uploaded.
Following on from the above, there really is no need or reason to upload any more data in your spreadsheet than is necessary, indeed there are strong arguments for uploading the bare minimum of data to meet HMRC's requirements.
Instead of uploading your whole accounts spreadsheet or data set, just upload a shortened summary version with the values in for the nine boxes on the VAT100 form. How you create this shortened version is entirely up to you (you shouldn't cut and paste the data though...) however the simplest method of uploading just the data would be to save the mtdvatbridge.uk interface sheet as a CSV file and upload that.