File Content Type

Subversion joins the ranks of the many applications that recognize and make use of Multipurpose Internet Mail Extensions (MIME) content types. Besides being a general-purpose storage location for a file’s content type, the value of the svn:mime-type file property determines some behavioral characteristics of Subversion itself.

For example, one of the benefits that Subversion typically provides is contextual, line-based merging of changes received from the server during an update into your working file. But for files containing nontextual data, there is often no concept of a “line.” So, for versioned files whose svn:mime-type property is set to a nontextual MIME type (generally, something that doesn’t begin with text/, though there are exceptions), Subversion does not attempt to perform contextual merges during updates. Instead, any time you have locally modified a binary working copy file that is also being updated, your file is left untouched and Subversion creates two new files. One file has a .oldrev extension and contains the BASE revision of the file. The other file has a .newrev extension and contains the contents of the updated revision of the file. This behavior is really for the protection of the user against failed attempts at performing contextual merges on files that simply cannot be contextually merged.

Warning

The svn:mime-type property, when set to a value that does not indicate textual file contents, can cause some unexpected behaviors with respect to other properties. For example, since the idea of line endings (and therefore, line-ending conversion) makes no sense when applied to nontextual files, Subversion will prevent you from setting the svn:eol-style property on such files. This is obvious when attempted on a single file target—svn propset will error out. But it might not be as clear if you perform a recursive property set, where Subversion will silently skip over files that it deems unsuitable for a given property.

Beginning in Subversion 1.5, users can configure a new mime-types-file runtime configuration parameter, which identifies the location of a MIME types mapping file. Subversion will consult this mapping file to determine the MIME type of newly added and imported files.

Also, if the svn:mime-type property is set, then the Subversion Apache module will use its value to populate the Content-type: HTTP header when responding to GET requests. This gives your web browser a crucial clue about how to display a file when you use it to peruse your Subversion repository’s contents.

Identifying File Types

Various programs on most modern operating systems make assumptions about the type and format of the contents of a file by the file’s name; specifically, its file extension. For example, files whose names end in .txt are generally assumed to be human-readable; that is, able to be understood by simple perusal rather than requiring complex processing to decipher. Files whose names end in .png, on the other hand, are assumed to be of the Portable Network Graphics type—not human-readable at all, and sensible only when interpreted by software that understands the PNG format and can render the information in that format as a raster image.

Unfortunately, some of those extensions have changed their meanings over time. When personal computers first appeared, a file named README.DOC would have almost certainly been a plain-text file, just like today’s .txt files. But by the mid-1990s, you could almost bet that a file of that name would not be a plain-text file at all, but instead a Microsoft Word document in a proprietary, non-human-readable format. But this change didn’t occur overnight—there was certainly a period of confusion for computer users over what exactly they had in hand when they saw a .DOC file.[10]

The popularity of computer networking cast still more doubt on the mapping between a file’s name and its content. With information being served across networks and generated dynamically by server-side scripts, there was often no real file per se, and therefore no filename. Web servers, for example, needed some other way to tell browsers what they were downloading so that the browser could do something intelligent with that information, whether that was to display the data using a program registered to handle that datatype or to prompt the user for where on the client machine it should store the downloaded data.

Eventually, a standard emerged for, among other things, describing the contents of a data stream. In 1996, RFC 2045 was published. It was the first of five RFCs describing MIME. It describes the concept of media types and subtypes and recommends a syntax for the representation of those types. Today, MIME media types—or “MIME types”—are used almost universally across email applications, web servers, and other software as the de facto mechanism for clearing up the file content confusion.


[10] You think that was rough? During that same era, WordPerfect also used .DOC for their proprietary file format’s preferred extension!

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