MM-64658 Fix handling of upload sessions (#32141)

* MM-64658 Fix handling of upload sessions

* Fix style issue

server/channels/api4/upload.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"mime/multipart"
 	"net/http"
+	"path/filepath"
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
@@ -39,6 +40,8 @@ func createUpload(c *Context, w http.ResponseWriter, r *http.Request) {
 	us.RemoteId = ""
 	us.ReqFileId = ""
 
+	us.Filename = filepath.Base(us.Filename)
+
 	auditRec := c.MakeAuditRecord("createUpload", model.AuditStatusFail)
 	defer c.LogAuditRec(auditRec)
 	model.AddEventParameterAuditableToAuditRec(auditRec, "upload", &us)