Check file uri sharing owner rather than prohibiting outright

Fixes #5381
// FREEBIE

jni/utils/org_thoughtcrime_securesms_util_FileUtils.cpp

@@ -0,0 +1,31 @@
+#include "org_thoughtcrime_securesms_util_FileUtils.h"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+jint JNICALL Java_org_thoughtcrime_securesms_util_FileUtils_getFileDescriptorOwner
+  (JNIEnv *env, jclass clazz, jobject fileDescriptor)
+{
+  jclass fdClass = env->GetObjectClass(fileDescriptor);
+
+  if (fdClass == NULL) {
+    return -1;
+  }
+
+  jfieldID fdFieldId = env->GetFieldID(fdClass, "descriptor", "I");
+
+  if (fdFieldId == NULL) {
+    return -1;
+  }
+
+  int fd = env->GetIntField(fileDescriptor, fdFieldId);
+
+  struct stat stat_struct;
+
+  if (fstat(fd, &stat_struct) != 0) {
+    return -1;
+  }
+
+  return stat_struct.st_uid;
+}