https://www.icyphoenix.com/viewtopic.php?f=35&t=3497&p=26311#p26311
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moreteavicar
Mon 17 Mar, 2008 12:49

Re: Couldn't Store Attachment
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Do you have phpMyAdmin? Go phpMyAdmin>>your database>>attachments_desc

Check that field 'attach_id' has auto-increment (under "Extra").
If not, select edit for that field (pencil icon), go across to the "Extra" drop-down menu and select "auto increment". 

It doesn't make sense that it shouldn't be auto-increment, perhaps some tables got corrupted, or you imported an SQL dump which didn't have full table structure...