From Valve Developer Community

sftowebm.exe is a tool that will convert Scaleform encoded WebM files to a standard WebM format which allows them to be played on media players and other tools such as HTML5 video players. Using this tool, third parties can convert the Dota 2 hero portrait Scaleform video files (and more) into a friendly usable WebM format.

You can download the binary from here.

Usage

To use the tool you will need to open up a command prompt window (Windows Key + R) and then point it to the directory containing the sftowebm.exe tool.

sftowebm.exe Convert a webmsf file to webm format Usage: sftowebm input.webm

You will need to place the Scaleform WebM file in the same directory as the tool and then (for example) run the following command:

sftowebm.exe npc_dota_hero_abaddon.webm

Upon doing so a second version of the file will be created in the same directory containing the extension ".webm.webm", this is the version that is formatted under the standard WebM format and will be playable on other devices and HTML5 video players.