From MozillaZine Knowledge Base
mimeTypes.rdf stores information about which action is to be performed when downloading certain types of files, such as opening the file in a specific program or saving it to disk. In Firefox, this information can be viewed or changed in "Tools -> Options -> Content / File Types -> Manage..."; in Thunderbird, in "Tools -> Options -> Attachments / Download Actions -> View & Edit Actions..."; and in Mozilla Suite and SeaMonkey, in "Edit -> Preferences -> Navigator -> Helper Applications".
In general, it is not advised to edit this file manually because of its complexity and the fact that interfaces are provided to edit it in Firefox, Thunderbird, and SeaMonkey. Note that build-in handlers or plug-ins may have precedence over the settings in mimeTypes.rdf.
The file is an RDF document containing one RDF:Seq and many RDF:Descriptions (one for each MIME type). Each entry has an identifier - the attribute RDF:about. To delete an entry, delete both its RDF:li in the RDF:Seq and its RDF:Description. To change an entry, simply change the RDF:Description's attributes (do not change RDF:about). To add an entry, insert a new RDF:li into the RDF:Seq and create a new RDF:Description, making sure their identifiers match.
These entries result in "Paint" always to be used as the application to open PNG images on Windows. While this works with Thunderbird, it wouldn't have any effect in SeaMonkey as it has an internal mechanism to handle PNG files. Only if the server requests explicitly a content disposition of "attachment", both Firefox and SeaMonkey will present the dialog proposing the defined action, regardless of the "NC:alwaysAsk" setting.
<?xml version="1.0"?> <RDF:RDF xmlns:NC="http://home.netscape.com/NC-rdf#" xmlns:RDF="http://www.w3.org/1999/02/22-rdf-syntax-ns#"> <RDF:Seq RDF:about="urn:mimetypes:root"> ... <RDF:li RDF:resource="urn:mimetype:image/png"/> ... </RDF:Seq> <RDF:Description RDF:about="urn:mimetype:image/png" NC:fileExtensions="png" NC:description="PNG Image" NC:value="image/png" NC:editable="true"> <NC:handlerProp RDF:resource="urn:mimetype:handler:image/png"/> </RDF:Description> <RDF:Description RDF:about="urn:mimetype:handler:image/png" NC:alwaysAsk="false" NC:saveToDisk="false" NC:useSystemDefault="false" NC:handleInternal="false"> <NC:externalApplication RDF:resource="urn:mimetype:externalApplication:image/png"/> </RDF:Description> <RDF:Description RDF:about="urn:mimetype:externalApplication:image/png" NC:path="C:\WINDOWS\system32\mspaint.exe" NC:prettyName="mspaint.exe" /> </RDF:RDF>
This file can be moved to a different profile. If some MIME types specify a path for a program to use, this path must be correct on the destination profile's system.
Deleting mimeTypes.rdf will clear the associations between MIME types and programs. A new file will be created on start up or when the application first needs to write to the file.