For the v2 Filestack API, if the file that is being uploaded is less than 1mb, then Filestack is transferring the file data via a standard ajax post request to the Filestack REST API.
If the file being uploaded is larger than 1mb then Filestack utilizes a multipart upload method. The file is first divided into 1mb chunks which are then uploaded to our system asynchronously. Filestack creates a maximum of 1000 chunks, so for bigger files the chunks may be proportionately larger. Once all of the file chunks have reached their destination, they are reassembled into a complete copy of the original file. Depending on the size of the file being uploaded, the re-assembly process can take a significant chunk of time. This process often begins at around the 85% mark of the upload progress bar. If you are dealing with large files, we urge you to be patient. When measured against other uploading techniques and services, large files are uploaded up to 20% faster when uploaded using Filestack.
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