The generate_presigned_put endpoint for direct external uploads
(such as the one for the uppy-image-uploader) records allowed
S3 metadata values on the uploaded object. We use this to store
the sha1-checksum generated by the UppyChecksum plugin, for later
comparison in ExternalUploadManager.
However, we were not doing this for the create_multipart endpoint,
so the checksum was never captured and compared correctly.
Also includes a fix to make sure UppyChecksum is the last preprocessor to run.
It is important that the UppyChecksum preprocessor is the last one to
be added; the preprocessors are run in order and since other preprocessors
may modify the file (e.g. the UppyMediaOptimization one), we need to
checksum once we are sure the file data has "settled".
99ec8eb6df