ASF File Extension

Advanced Systems Format: ASF structured file that contains audio and/or video content compressed with 3rd party codecs 
File extension: .asf
MIME type: video/x-ms-asf
Type code: 'ASF_'
Uniform Type Identifier: com.microsoft.advanced-​systems-format
Magic: 30 26 b2 75
Developed by: Microsoft
Type of format: Container format
Container for: WMA, WMV, MPEG4 etc.

MP3 File Extension

MPEG-1 Audio Layer 3
File extension: .mp3
MIME type: audio/mpeg
Type of format: Audio

Convert ASF to MP3 with Mini-stream Ripper or ASX to MP3 Converter