When exporting a file from a client using the DownloadFromStream method in a Business Central online environment, you need to create an InStream data type to send the data. The solution is to use the CreateInStream method from codeunit "Temp Blob" (A). The Temp Blob codeunit provides temporary storage of BLOBs (Binary Large Objects) and is commonly used for handling files and streams in Business Central. By using the CreateInStream method on a Temp Blob, you create an InStream that can then be used with the DownloadFromStream method to send the file data to the client. This approach is efficient for file handling and transfer in Business Central, especially in scenarios involving data export or file manipulation.