When resetting the HTTP File Upload plugin any pending files (files that have been sent but not accepted by the receiver yet) are deleted as they are kept in cache and then they can’t be delivered anymore. It would be nice if unsent files are not removed when the plugin is reset.
By default, the plugin will store data in a temporary directory, that is removed when things are restarted That is probably the cause of your issue.
The plugin readme documents how you can change this:
|XML property||System Property|
By default, the plugin will store all uploaded files in a temporary directory that is provided by the Operating System. This directory is configured to be deleted, after Openfire quits. Some Operating Systems are known to periodically purge these temporary directories, which is another potential source of data loss. This plugin can be configured to use any other directory for data storage, by providing the path for that directory in either property listed above. Note that the directory is expected to exist (this plugin will not attempt to create it if it is missing). Be sure that the process owner of the Openfire process has enough permissions to read and write in that directory.