In this article I will explain with an example, how to make FileUpload control accept only CSV file in ASP.Net.
The accept attribute is a new attribute available in HTML5.
Note: Accept Attribute does not restrict user from uploading invalid file.
HTML Markup
The HTML Markup consists of:
FileUpload – For selecting file.
The FileUpload control has been set with the following attribute.
accept – This attribute allows user to select only specified type of files from the Choose File dialog box.
Note: Accepting all files is default behaviour and hence even if the accept attribute is not specified, by default, it will accept all files.
For example, in the following code, the FileUpload control will only allow user to select CSV file i.e. file with extension csv.
<asp:FileUpload runat="server" accept=".csv" />
DO NOT use the accept attribute for validation
The accept attribute does not restrict user from uploading invalid files.
FileUpload Control accept CSV file in ASP.Net
And hence, if you want to restrict user and perform validation then please refer the following articles.
FileUpload Control accept CSV files in ASP.Net
Browser Compatibility
The above code has been tested in the following browsers only in versions that support HTML5.
Microsoft Edge  FireFox  Chrome  Safari  Opera
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