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.
closed as too broad by Evil Closet Monkey, Charles Wesley, Graham Herrli, Bart Gijssens, Matt Obee Oct 21 '14 at 8:32
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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.