Many mobile apps and desktop editors (like iMovie or Windows Photos) use this naming convention if you don't provide a custom title. For example, Gifu Seiki Industrial uses this file name for technical demonstration clips on their corporate site.
Upload the file or the URL to a service like VirusTotal to check for hidden scripts. MyVideo_1mp4
MyVideo_1 | モノづくりプランナー - 岐阜精機工業株式会社 Many mobile apps and desktop editors (like iMovie