Editing Email attachment

{{Short description|Computer file sent along with an email}}
An '''email attachment''' is a [[computer file]] sent along with an [[Email|email message]]. One or more files can be attached to any email message, and be sent along with it to the recipient. This is typically used as a simple method to share documents and images.

==History, and technical detail==
Originally, ARPANET, UUCP, and Internet [[SMTP]] email allowed [[ASCII|7-bit ASCII]] text only.  Text files were emailed by including them in the message body.  In the mid 1980s text files could be grouped with [[UNIX]] tools such as bundle<ref>The UNIX Programming Environment, Kernighan and Pike, 1984, p.97</ref><ref>{{cite journal|title=Unix tricks and traps|journal=AUUGN|date=August 1994|volume=15|issue=4|page=87|url=https://books.google.com/books?id=fNSEwqH75t0C&q=shar}}</ref> and [[Shar (file format)|shar]] (shell archive)<ref>Modern versions of [[Shar (file format)|shar]] can deal with binaries, via uuencoding them, but this was not initially the case.</ref> and included in email message bodies, allowing them to be unpacked on remote UNIX systems with a single shell command.

The COMSYS/MSGDMS system at MIT offered "Enclosures" beginning by 1976.<ref>[http://mercury.lcs.mit.edu/~jnc/tech/header/mins02.txt "Jack Haverty, email to Header-People, 8 November 1976"]</ref><ref>[https://www.computerhistory.org/collections/catalog/102806104 "Feinler, Vittal: Email Innovation Timeline, 1 July 2022"]</ref> Users inside COMSYS could receive the enclosure file directly. Messages sent to users out of the COMSYS world sent the enclosure as part of the message body, which was useful only for text files.

Attaching non-text files was first accomplished in 1980 by manually encoding 8-bit files using [[Mary Ann Horton]]'s [[uuencode]], and later using [[BinHex]] or [[xxencode]]<ref>[http://kb.winzip.com/kb/?View=entry&EntryID=103 ''"How do I use UUencode/BinHex/MIME support?"'', winzip.com].</ref> and pasting the resulting text into the body of the message.  When the "Attachment" user interface first appeared on PCs in [[cc:Mail]] around 1985,<ref>{{cite book|author=InfoWorld Media Group, Inc.|title=InfoWorld|url=https://books.google.com/books?id=8C4EAAAAMBAJ&pg=PA41|date=June 3, 1985|publisher=InfoWorld Media Group, Inc.|page=41}}</ref> it used the [[uuencode]] format for SMTP transmission, as did [[Microsoft Mail]] later.

Modern email systems use the [[MIME]] standard, making email attachments more utilitarian and seamless. This was developed by [[Nathaniel Borenstein]] and collaborator [[Ned Freed]]<ref name=kingsley>[https://www.theguardian.com/technology/2012/mar/26/ather-of-the-email-attachment Father of the email attachment], Patrick Kingsley, ''The Guardian'', 26 March 2012</ref><ref>[http://www.networkworld.com/news/2011/020111-mime-internet-email.html?page=1 "The MIME guys: How two Internet gurus changed e-mail forever "] {{Webarchive|url=https://web.archive.org/web/20120125015429/http://www.networkworld.com/news/2011/020111-mime-internet-email.html?page=1 |date=2012-01-25 }}, February 01, 2011, Jon Brodkin, Network World</ref> - with the standard being officially released as [http://tools.ietf.org/html/rfc2045 RFC2045] in 1996.

With MIME, a message and all its attachments are encapsulated in a single [[multipart message]], with [[base64]] encoding used to convert binary into 7-bit ASCII text - or on some modern mail servers, optionally [[8-bit clean|full 8-bit support]] via the [[8BITMIME]] extension.

==Size limits==
Email standards such as [[MIME]] do not specify any file size limits, but in practice, email users will find that they cannot successfully send very large files across the Internet.

This is because of a number of potential limits: 
* Mail systems often arbitrarily limit the size their users are allowed to submit.<ref>[http://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html ''"Setting Message Size Limits in Exchange 2010 and Exchange 2007"''];</ref> 
* A message will often pass through several [[mail transfer agent]]s to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore also impose size limits. 
* The recipient mail system may reject incoming emails with attachments over a certain size.

The result is that while large attachments may succeed internally within a company or organization, they may not when sending across the Internet.

As an example, when [[Google]]'s [[Gmail]] service increased its arbitrary limit to 25MB it warned that: "''you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits''".<ref>[http://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629/#ixzz0oIzFY0Q8 ''"Google updates file size limits for Gmail and YouTube"'', geek.com] {{Webarchive|url=https://web.archive.org/web/20111219141547/http://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629#ixzz0oIzFY0Q8 |date=2011-12-19 }}.</ref><ref>[http://mail.google.com/support/bin/answer.py?answer=8770&topic=1517 ''"Maximum attachment size"'', mail.google,com].</ref> 

Also note that all these size limits are based, not on the original file size, but the [[MIME]]-encoded copy. The common [[Base64]] encoding adds about 37% to the original file size, meaning that an original 20MB file could exceed a 25MB file attachment limit.<ref>{{cite web|url=http://luxsci.com/blog/raw-vs-encoded-email-message-size-whats-the-difference.html|title=Raw vs. Encoded Email Message Size — What's the Difference?}}</ref>
A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.

Users should be cautious with certain file formats when received as email attachments, such as [[ZIP (file format)|.zip]] and [[.tgz]] files, because they can contain harmful viruses and potential software. .iso files can also be used to spread malware and [[.exe]] is an executable file that can become active on a computer as soon as it is opened.

==Malware==
{{See also|Spamming}}
A lot of [[malware]] is distributed via email attachments with some even considering such to be the main [[Vector (malware)|vector]] for cyberattacks on businesses.<ref name=techa>{{cite web|last1=Martin|first1=Jim|title=Here's what you need to do to protect your PC from ransomware and NotPetya|url=http://www.techadvisor.co.uk/feature/security/what-is-ransomware-how-do-i-protect-my-pc-from-petya-3659036/|publisher=Tech Advisor|access-date=29 June 2017}}</ref><ref>{{cite web|title=Truth on zero-day attacks|url=https://www.pcr-online.biz/news/read/truth-on-zero-day-attacks/039452|publisher=PCR|access-date=29 June 2017}}</ref><ref>{{cite book|last1=Aycock|first1=John|title=Computer Viruses and Malware|date=2006|publisher=Springer|isbn=9780387341880|url=https://books.google.com/books?id=xnW-qvk1gzkC&pg=PA142|access-date=29 June 2017|language=en}}</ref> Users are advised to be extremely cautious with attachments and to not open any attachments that are not from a trusted source and expected<!--by the recipient--> − even if the sender is in their address book as their account might have been taken over or misused.<ref name=techa/><ref>{{cite book|last1=Miller|first1=Michael R.|title=Microsoft Security Essentials User Manual (Digital Short Cut)|date=2009|publisher=Pearson Education|isbn=9780768695298|url=https://books.google.com/books?id=wVAfzSIMD-4C&pg=PT95|access-date=29 June 2017|language=en}}</ref><ref>{{cite book|last1=Vermaat|first1=Misty E.|title=Enhanced Discovering Computers, Essentials|date=2014|publisher=Cengage Learning|isbn=9781285845531|url=https://books.google.com/books?id=XBIeCgAAQBAJ&pg=PA23|access-date=29 June 2017|language=en}}</ref> While many email servers scan attachments for malware and block dangerous filetypes, this should not be relied upon − especially as such cannot detect [[Zero-day (computing)|zero-day exploits]].<ref>{{cite web|title=How To Spot A Dangerous Email Attachment|url=http://www.makeuseof.com/tag/spot-dangerous-email-attachment/|publisher=MakeUseOf|access-date=29 June 2017}}</ref> 

===Dangerous file types===
Email users are typically warned that unexpected email with attachments should always be considered suspicious and dangerous, particularly if not known to be sent by a trusted source. However, in practice this advice is not enough – "known trusted sources" were the senders of [[Computer program|executable programs]] creating mischief and mayhem as early as 1987 with the mainframe-based [[Christmas Tree EXEC]].

Since the [[ILOVEYOU]] and [[Anna Kournikova (computer virus)|Anna Kournikova]] [[Computer worm|worms]] of 2000 and 2001, email systems have increasingly added layers of protection to prevent potential [[malware]]. Now, many block certain types of attachments.<ref>[http://mail.google.com/support/bin/answer.py?hl=en&answer=6590 ''"Some file types are blocked"'', mail.google.com].</ref><ref>[http://support.microsoft.com/kb/829982 ''"You may receive an "Outlook blocked access to the following potentially unsafe attachments" message in Outlook"'', microsoft.com].</ref>

==References==
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[[Category:Email]]