It could be a document or a video.
What Is an AV File?
Some files that use the AVfile extensionare documents created by Final Draft AV version 1.

Later editions create documents with the .XAV file extension instead.
Template files use the similar XAVT file format.
AV files are used by the program to store this information.
If your AV file isn’t a document, it’s most likely a video.
Some cameras use this file extension for storing video data.
AV (or A/V) also stands for “audio/visual” when referring to composite and component AV cables.
The only software we know that might support video files that use the .AV file extension isYoosee.
Yoosee, linked above, should be able to convert from AV to MP4.
From there, you might use afree video converterto save the MP4 to another preferred video format.
Still Can’t pop launch the File?
You might be misreading the file extension, which is remarkably easy to do, especially with two-letter extensions.
MAV is another to watch out for.
This could easily be confused for an AV file.
You need Microsoft Access to open it.
FAQ
Windows Media Player doesn’t support files with the AV extension.
Although AV serves a similar purpose, the formats are compatible with different programs.