When I uploaded a file to SharePoint it gets modified, which is apparently intended for office files according to this Blog. This only happens to .dwt files which also seem to be the extension for Frontpage files (SP also displays a Frontpage-like icon next to them). The problem is, these DWT files are no office files but Auto CAD files.
After lots of research I found out that it only happens after I update the additional required fields of the Listitem. So first I upload a file, then the Metadata dialog pops up. If I download the file now it is fine but after I click save on the dialog the filesize increases. Looks like SharePoint appends the Metadata to the file.
So why is SharePoint doing this when it is no real office file and thus corrupts it? And more importantly how can I prevent this behaviour?
I see that there are already similar questions here but they are not applicable to my problem as it seems. There is a 3rd party tool installed (lookupfieldwithpicker) but only in the main WebApp. I also tested this in a completely new WebApp without custom code and the same problem arises.
Update: Is there any information to this from Microsoft, like a "known issue"? If I change the file extension to something else, the file stays untouched and is fine.
Update 2: I forgot to mention, this is SharePoint 2010 (don't know if this happens in other versions too).