I think there're two:
OP's proposal is to automatically use the filename as ALT text instead of "image" as a default. How to do it (without javascript I assume) is the question. Maybe extracting it before is posted, but for old posts may be tricky.
After that others pointed out that an [img="{URL}"]{ALT_TEXT}[/img] bbcode also made sense in case you want to override the default. Technically you can do It by yourself as a custom bbcode but I can't use [img] so I had to create [image=""] which is unnecesary inconsistent.
Provide better ALT Names on images.
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Re: Provide better ALT Names on images.
Slightly better English than it was in 2005, still improving