Steps to reproduce:
1. Take an m4v or webm video file and put it into StreamingAssets folder
2. Take it out to any other folder
3. It remains as an unspecified video file
4. If you add the same file straight to assets and anywhere but StreamingAssets folder it works fine and is interpreted correctly

Expected: Once you take out these video files out, you should have a choice as with other video formats to have it as "Movie Texture" or "Video Clip"
Actual: After you take it out you get an error

Reproduced on: 2018.1.0a6, 2017.3.0b11, 2017.2..0p4, 2017.1.2p3

Assertion failed: GetObject failed to find Object for Library Representation: name 'pandemic-tlr1_i320' path 'Assets/pandemic-tlr1_i320.m4v' guid 473b97371b9885449af143332991d9a4
UnityEditorInternal.InternalEditorUtility:ProjectWindowDrag(HierarchyProperty, Boolean)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)