diff --git a/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/workouts/WorkoutDetailsFragment.kt b/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/workouts/WorkoutDetailsFragment.kt
index ae370d335c..cb6fda1f59 100644
--- a/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/workouts/WorkoutDetailsFragment.kt
+++ b/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/workouts/WorkoutDetailsFragment.kt
@@ -750,10 +750,15 @@ class WorkoutDetailsFragment : Fragment(), MenuProvider {
         val workoutName = currentWorkout!!.summary.name
         val activityKind = ActivityKind.fromCode(currentWorkout!!.summary.activityKind)
         val activityTrackProvider = gbDevice.deviceCoordinator.getActivityTrackProvider(gbDevice, requireContext())
-        val gpxFile = ActivitySummaryUtils.getShareableGpxFile(activityTrackProvider, workout.summary)
 
-        if (gpxFile == null) {
-            GB.toast(getString(R.string.no_gpx_track_in_activity_toast), Toast.LENGTH_LONG, GB.INFO)
+        val activityFile = if (workout.summary.rawDetailsPath?.endsWith(".fit") == true) {
+            FileUtils.tryFixPath(workout.summary.rawDetailsPath)
+        } else {
+            ActivitySummaryUtils.getShareableGpxFile(activityTrackProvider, workout.summary)
+        }
+
+        if (activityFile == null) {
+            GB.toast(getString(R.string.no_activity_track_in_activity_toast), Toast.LENGTH_LONG, GB.INFO)
             return
         }
 
@@ -763,7 +768,7 @@ class WorkoutDetailsFragment : Fragment(), MenuProvider {
             val apiClient = EndurainApiClient(serverUrl!!, endurainVm.tokenManager)
             endurainVm.tokenManager.performTokenRefresh(serverUrl) {
                 LOG.info("Uploading workout '{}' (type {}) to Endurain", workoutName, activityKind)
-                apiClient.uploadActivity(gpxFile) { newId ->
+                apiClient.uploadActivity(activityFile) { newId ->
                     if (newId != null) {
                         // Update activity type on the server
                         apiClient.editActivity(newId, activityKind, workoutName)
diff --git a/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/InternetUtils.kt b/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/InternetUtils.kt
index 3c7c893203..90451f39c6 100644
--- a/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/InternetUtils.kt
+++ b/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/InternetUtils.kt
@@ -171,17 +171,17 @@ class InternetUtils {
                 }
 
                 val boundary = "----GadgetbridgeFormBoundary${System.currentTimeMillis()}"
-                val multipartBody = buildMultipartBody(file, fileName, mimeType, boundary)
+                val multipartBodyBytes = buildMultipartBody(file, fileName, mimeType, boundary)
 
                 val headers = requestHeaders.toMutableMap()
                 headers["Content-Type"] = "multipart/form-data; boundary=$boundary"
 
                 val response = if (GBApplication.hasDirectInternetAccess()) {
-                    directRequest(
+                    directBinaryRequest(
                         uri = uri,
                         method = "POST",
                         requestHeaders = headers,
-                        body = multipartBody,
+                        body = multipartBodyBytes,
                         allowInsecure = allowInsecure
                     )
                 } else {
@@ -189,7 +189,7 @@ class InternetUtils {
                         uri,
                         HttpRequest.Method.POST,
                         headers,
-                        multipartBody.toByteArray(),
+                        multipartBodyBytes,
                         allowInsecure
                     )
                 }
@@ -207,24 +207,57 @@ class InternetUtils {
             fileName: String,
             mimeType: String,
             boundary: String
-        ): String {
+        ): ByteArray {
             val fileBytes = file.readBytes()
             val output = ByteArrayOutputStream()
-            val writer = output.bufferedWriter()
-
-            writer.write("--$boundary\r\n")
-            writer.write("Content-Disposition: form-data; name=\"file\"; filename=\"$fileName\"\r\n")
-            writer.write("Content-Type: $mimeType\r\n")
-            writer.write("\r\n")
-            writer.flush()
 
+            val header = "--$boundary\r\n" +
+                "Content-Disposition: form-data; name=\"file\"; filename=\"$fileName\"\r\n" +
+                "Content-Type: $mimeType\r\n" +
+                "\r\n"
+            output.write(header.toByteArray(Charsets.US_ASCII))
             output.write(fileBytes)
+            output.write("\r\n--$boundary--\r\n".toByteArray(Charsets.US_ASCII))
 
-            writer.write("\r\n")
-            writer.write("--$boundary--\r\n")
-            writer.flush()
+            return output.toByteArray()
+        }
 
-            return output.toString("UTF-8")
+        /**
+         * Direct HTTP request using OkHttp with a binary body.
+         */
+        private fun directBinaryRequest(
+            uri: Uri,
+            method: String,
+            requestHeaders: Map<String, String>,
+            body: ByteArray,
+            allowInsecure: Boolean
+        ): WebResourceResponse {
+            val client = if (allowInsecure) createInsecureClient() else defaultClient
+            val builder = Request.Builder().url(uri.toString())
+
+            for ((key, value) in headersWithUserAgent(requestHeaders)) {
+                builder.addHeader(key, value)
+            }
+
+            val contentType = getHeader(requestHeaders, "content-type") ?: "application/octet-stream"
+            val requestBody = body.toRequestBody(contentType.toMediaType())
+            builder.method(method.uppercase(), requestBody)
+
+            client.newCall(builder.build()).execute().use { response ->
+                val statusCode = response.code
+                val message = if (!response.message.isEmpty()) response.message else "OK"
+                val headers = response.headers.toMap()
+                val respContentType = response.header("content-type") ?: "application/octet-stream"
+                val encoding = response.header("content-encoding") ?: "UTF-8"
+                return WebResourceResponse(
+                    respContentType,
+                    encoding,
+                    statusCode,
+                    message,
+                    headers,
+                    ByteArrayInputStream(response.body.bytes())
+                )
+            }
         }
 
         /**
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 1a35735053..30c223ee18 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -4970,6 +4970,7 @@
     <string name="pref_endurain_log_out_title">Log out of Endurain</string>
     <string name="pref_endurain_log_out_summary">Tap here to log out, this disables the integration</string>
     <string name="no_gpx_track_in_activity_toast">No GPX track in this activity</string>
+    <string name="no_activity_track_in_activity_toast">No activity track in this activity</string>
     <string name="unable_to_display_gpx_track_toast">Unable to display GPX track: %1$s</string>
     <string name="unable_to_share_gpx_track_toast">Unable to share GPX track: %1$s</string>
     <string name="endurain_successfully_uploaded_toast">Successfully uploaded to Endurain</string>