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Through the MultiUpload feature, you are able to upload up to 20 files per activity having each one 200 MB as max size.
You can change the default behavior through exo.properties file by configuring these two parameters:
exo.social.composer.maxToUpload=20, default value set to 20.
exo.social.composer.maxFileSizeInMB=200, default value set to 200 MB.