Add VIDEO_FILE_EXTENSIONS into the "default" set

[yarikoptic]

part of #918 by @Saksham20 . to avoid needing DANDI_DEVEL modes of operation on video files, we should treat them as 1st class citizens for organize and upload operations

[jwodder]

@yarikoptic In what way are they not "first class citizens" for organize? What behavior change is needed there?

[yarikoptic]

#841 by @Saksham20 I believe made it possible to organize NWB + video files into a dandiset. So there should be no need for DANDI_DEVEL=1 switch to make organize consider them... if that is not the case already -- then may be no more need to be done and they just need to become "first class citizen" for all other commands which have to deal with them, e.g. upload