diff --git a/README.md b/README.md
index 122bde1..11fb8ba 100644
--- a/README.md
+++ b/README.md
@@ -57,11 +57,16 @@ logs.)
 * `log`: a log file, with lines separated by newline characters. Multiple log
   files can be included by including several `log` parts.
 
+  If the log is uploaded with a filename `name.ext`, where `name` contains only
+  alphanumerics, `.`, `-` or `_`, and `ext` is one of `log` or `txt`, then the
+  file saved to disk is based on that. Otherwise, a suitable name is
+  constructed.
+
   If using the JSON upload encoding, the request object should instead include
   a single `logs` field, which is an array of objects with the following
   fields:
 
-    * `id`: textual identifier for the logs. Currently ignored.
+    * `id`: textual identifier for the logs. Used as the filename, as above.
     * `lines`: log data. Newlines should be  encoded as `\n`, as normal in JSON).
 
 * `compressed-log`: a gzipped logfile. Decompressed and then treated the same as
diff --git a/src/github.com/matrix-org/rageshake/submit.go b/src/github.com/matrix-org/rageshake/submit.go
index 461c150..8af6ebd 100644
--- a/src/github.com/matrix-org/rageshake/submit.go
+++ b/src/github.com/matrix-org/rageshake/submit.go
@@ -189,7 +189,7 @@ func parseJSONRequest(w http.ResponseWriter, req *http.Request, reportDir string
 
 	for i, log := range p.Logs {
 		buf := bytes.NewBufferString(log.Lines)
-		leafName, err := saveLogPart(i, buf, reportDir)
+		leafName, err := saveLogPart(i, log.ID, buf, reportDir)
 		if err != nil {
 			return nil, err
 		}
@@ -286,7 +286,7 @@ func parseFormPart(part *multipart.Part, p *parsedPayload, reportDir string) err
 	}
 
 	if field == "log" || field == "compressed-log" {
-		leafName, err := saveLogPart(len(p.Logs), partReader, reportDir)
+		leafName, err := saveLogPart(len(p.Logs), part.FileName(), partReader, reportDir)
 		if err != nil {
 			return err
 		}
@@ -359,11 +359,27 @@ func saveFormPart(leafName string, reader io.Reader, reportDir string) (string,
 	return leafName, nil
 }
 
+// we require a sensible extension, and don't allow the filename to start with
+// '.'
+var logRegexp = regexp.MustCompile(`^[a-zA-Z0-9_-][a-zA-Z0-9_.-]*\.(log|txt)$`)
+
 // saveLogPart saves a log upload to the report directory.
 //
 // Returns the leafname of the saved file.
-func saveLogPart(logNum int, reader io.Reader, reportDir string) (string, error) {
-	leafName := fmt.Sprintf("logs-%04d.log.gz", logNum)
+func saveLogPart(logNum int, filename string, reader io.Reader, reportDir string) (string, error) {
+	// pick a name to save the log file with.
+	//
+	// some clients use sensible names (foo.N.log), which we preserve. For
+	// others, we just make up a filename.
+	//
+	// Either way, we need to append .gz, because we're compressing it.
+	var leafName string
+	if logRegexp.MatchString(filename) {
+		leafName = filename + ".gz"
+	} else {
+		leafName = fmt.Sprintf("logs-%04d.log.gz", logNum)
+	}
+
 	fullname := filepath.Join(reportDir, leafName)
 
 	f, err := os.Create(fullname)
diff --git a/src/github.com/matrix-org/rageshake/submit_test.go b/src/github.com/matrix-org/rageshake/submit_test.go
index 31ce798..db4c895 100644
--- a/src/github.com/matrix-org/rageshake/submit_test.go
+++ b/src/github.com/matrix-org/rageshake/submit_test.go
@@ -112,7 +112,7 @@ func TestMultipartUpload(t *testing.T) {
 
 	// check logs uploaded correctly
 	checkUploadedFile(t, reportDir, "logs-0000.log.gz", true, "log\nlog\nlog")
-	checkUploadedFile(t, reportDir, "logs-0001.log.gz", true, "log")
+	checkUploadedFile(t, reportDir, "console.0.log.gz", true, "log")
 	checkUploadedFile(t, reportDir, "logs-0002.log.gz", true, "test\n")
 
 	// check file uploaded correctly
@@ -156,7 +156,7 @@ log
 log
 log
 ------WebKitFormBoundarySsdgl8Nq9voFyhdO
-Content-Disposition: form-data; name="log"; filename="instance-0.067644760733513781492004890379"
+Content-Disposition: form-data; name="log"; filename="console.0.log"
 Content-Type: text/plain
 
 log
@@ -207,7 +207,7 @@ func checkParsedMultipartUpload(t *testing.T, p *parsedPayload) {
 	if p.Logs[0] != wanted {
 		t.Errorf("Log 0: got %s, want %s", p.Logs[0], wanted)
 	}
-	wanted = "logs-0001.log.gz"
+	wanted = "console.0.log.gz"
 	if p.Logs[1] != wanted {
 		t.Errorf("Log 1: got %s, want %s", p.Logs[1], wanted)
 	}