Unity projects targeting the 2.0/3.5 runtime or built with mono < v5.0 generate `mdb` files, not `pdb` files.
Looks like the `crashlytics-build.properties` gets around in more than just the `StreamingAssets` folder, looking at [examples around the internets](https://github.com/auth0/sharelock-android/blob/master/app/src/main/assets/crashlytics-build.properties), so it should probably just be ignored as a filename.
*.booproj
*.svd
*.pdb
+*.mdb
*.opendb
*.VC.db
# Unity3D generated meta files
*.pidb.meta
*.pdb.meta
+*.mdb.meta
# Unity3D generated file on crash reports
sysinfo.txt
*.unitypackage
# Crashlytics generated file
-Assets/StreamingAssets/crashlytics-build.properties
+crashlytics-build.properties