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Editorial Team
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Asked: 2026-05-28T00:23:03+00:00

I have a simple web form that sends and email out via .NET C#.

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I have a simple web form that sends and email out via .NET C#. Additionally there is an ASP UPLOAD ctrl from which I create a memory stream and attach the stream to the Mail Message…ie no need to save file to file system.

ISSUE: When MAC users dont inlcude extensions on their uploaded files (I am testing for PostedFile.ContentType to determine the file and/or appropriate extension, allowable files RTF, TXT, PDF, DOC, DOCX) it seems like the ContentType comes back as “application/octet”…this makes some sense to me w/o an extension as required by PCs but; still unclear on weather reading the ContentType or extension is a better way to determine what type of file it is before I attach. [Extensionless MAC file will obviously fail any extension test]

Basically, I read the file from the upload ctrl, determine what its ContentType is, strip just the PostedFile.Filename and then reassemble as I create the attachment using the ContentType string in a switch statement to determine and add the correct extension to the attached file (filename + ext). I did this in order to ensure MAC users that dont use extensions can still upload w/o creating a .ZIP or other package.

Question 1: Is my logic sound with regards to approaching this issue, best practices?
Question 2: Will all MAC files return as “application/octect” when not using an extension on MAC, and if so, how can someone determine what type of file it really is? (Note: PDF worked fine when not using extensions on MAC platform).

Thanks in advance.
If code sample is necessary please advise.

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1 Answer

  1. Editorial Team

    Dont use File Extensions to work out MIME Types, instead use “Winista” for binary analysis, see my method here that resorts back to URLMon if winista cant work it out last answer
    http://social.msdn.microsoft.com/Forums/en-US/Vsexpressvcs/thread/d79e76e3-b8c9-4fce-a97d-94ded18ea4dd

    Say someone renames a exe with a jpg extension… you can still determine the “real” file format. It doesn’t detect swf’s or flv’s but does pretty much every other well known format + you can get a hex editor and add more files it can detect.

    Edit:
    Download Winista: http://www.netomatix.com/Products/DocumentManagement/MimeDetector.aspx

    This question has given me a great idea, since you wish to append a File Extension that will safely work on MACs and PCs I/you could edit the mime-types.xml file to include safe file extensions that will work to open specific file types on both MAC and PCs.

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