What would be a good wording and place on the form for file type error message? I am especially interested in the case when a user may not understand clearly the difference between file types.
The placement should be where the user is looking (the submit button and the form). The fact that something went wrong should be made VERY clear: the form changes color, etc...
Re the wording - I'm assuming you're talking about non-techies who are asked to send PDFs (as for example) and send word docs or something else instead. The error msg itself should be written in a non-techie way.
"I'm sorry but I can only read files saved as a PDF. This file was saved as a Microsoft Word document. Please go back to the word doc and "Save As" a PDF and then try uploading again.