Once you have selected the file for uploading, our app will remember what size it is so it can calculate the total size of your transfer.
However, if you select a file for uploading while it's still being edited (for example: being copied to your computer, before saving it, or before moving it to a different location), the app does not get the correct information about the file size.
To avoid this, don't fiddle with your files after selecting them for a transfer.
We've seen an increase in these errors for transfers with .mov files in them. Please make sure the .mov is completely rendered, is not open in the background and is actually done copying to your computer before select the file for your transfer.
An easy way to ensure this, is by creating a .zip file from all of the files together and upload that instead.