I have no solution for you, but yeah, after updating to latest Smack library Spark is having problems in some cases with file transfer (it still works in some other cases). [SPARK-1869] Degraded file transfer feature when sending files between different networks - IgniteRealtime JIRA
There are no active regular java developers working on Spark, so it can be years or never. Alternative solution would be using Spark 2.7.7 or some other client if file transfer is critical for you (you can download any version of Spark by changing version number in the download link).