A few words on file types are in order. You should be aware that not all format of files can be re-loaded into Dia after saving and that most supported file formats lose information to some degree.
The only format guaranteed to be lossless is Dia XML. Happily, you are unlikely to accidentally save in a lossy format because → always produces a file in Dia XML format.
Files in Dia XML format are automatically compressed using gzip to save disk space. This is almost necessary since, without compression, even simple diagrams are very large because they store much redundant information. Dia XML files are already compressed, so further compressing them using other archive formats will not help make them significantly smaller.
Dia is able to import and export the following file types:
Scalable Vector Graphics (.svg)
AutoCad Drawing eXchange Format (.dxf)
Visio XML File Format (.vdx)
XFig File Format (.fig)
GdkPixbuf bitmap graphics (.bmp, .gif, .jpg, .png, .pnm, .ras, .tif)
Dia supports exporting to many other types of file, such as:
Computer Graphics Metafile (.cgm)
Dia Shape File (.shape)
HP Graphics Language (.plt, .hpgl)
Encapsulated Postscript (.eps, .epsi)
Postscript (.ps)
Portable Document Format (.pdf)
Portable Network Graphics (.png)
TeX Metapost macros (.mp)
TeX PSTricks macros (.tex)
WordPerfect Graphics (.wpg)
XFig format (.fig)
XSLT (eXtensible Stylesheet Language Transformation) (.code)
Dia cannot load files in most of these formats. These formats are useful for diagrams that you need to use in another application or to distribute to users of another operating system that Dia does not support.
To create a file in one of these formats, select → from the Main Popup Menu. Choose a name and the desired format in the resulting dialog, which works like a save dialog. An appropriate file extension is automatically added.