Having problems with the new uploader system? Does it get "stuck" or otherwise not work for you? Things to try:
1. Make sure you uploaded all the WordPress files correctly. Really, this is probably the most common upgrade problem. If you're getting any sort of "Call to undefined function" message from the uploader, then this is absolutely your problem. Upload ALL the files. Don't "overwrite if newer". Delete wp-admin and wp-includes from the server and upload them fresh from the downloaded files. Really, we're not making this up, and everybody seems to just skip this step. I would even go so far as to suggest that you redownload the 2.5 zip file from the download link above. Some people seem to have downloaded the wrong files or had bad downloads that they didn't notice.
2. Clear your browser cache, restart your browser. Do it twice. Browser caches can be persistent little things. Also remove any files from the /wp-content/uploads/js_cache directory on your site. This may fix problems with the visual editor as well.
3. Update your Adobe Flash to the latest version (esp you Linux users!): Flash
4. Another known issue is where the media buttons don't appear at all. This happens when you have an older plugin interfering with the JavaScript. I encountered this problem with an older version of WP-Amazon, but any plugin loading its own jQuery can cause the issue. Disable your plugins and see if the buttons appear. If so, reactivate one by one until they don't appear any more. Upgrade all your plugins as well.
5. The highly popular Bad Behavior plugin breaks the uploader. Upgrade it to 2.0.14 (or later):
http://wordpress.org/extend/plugins/bad-behavior/
6. mod_security might be causing problems. Disable it to see if that is the problem. To do this, make an .htaccess file in your wp-admin directory. Add this to it:
<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>That will disable mod_security for the upload file receiving code in the wp-admin directory.
7. Try a smaller image, in pixels. Filesize is largely irrelevant, it's the width and height of the image that matters. Smaller images require less RAM. Although WordPress does attempt to tell PHP to allow it enough RAM, not all server configurations allow this and so PHP may not be set to have enough memory to work with larger images. Resize the images to web-sizes before uploading them.
8. Q: It uploads without any errors, but it does not generate thumbnails! The images show up full size on the page, or they are scaled in a really ugly manner.
A: In all cases I have seen this, the problem turned out to be with the image itself. WordPress uses the normal PHP functions for working with images, which are in the "GD" library. GD has problems with certain types of images. One particular image type that it cannot handle is grayscale images. If your image is black and white or grayscale, then make sure that it's a FULL COLOR image. That is, 24 or 32 bits per pixel. IrfanView is particularly good at converting images up from 256 color grayscale to 16 million color images. Just load the image up, select Image->Increase Color Depth, then resave it. Then upload the image.
9. Q: The gallery's align left/center/right selection box doesn't work!
A: Put this or something similar into your theme. The images will then align correctly on the page, albeit not in the visual editor.
.aligncenter { display:block; margin-left:auto; margin-right:auto; }
.alignleft { display:block; margin-right:auto; }
.alignright { display:block; margin-left:auto; }---
Finally, if you simply want to give up entirely on the flash uploader for now, use this plugin to disable it: No-Flash-Uploader
But most importantly, stay tuned. If any new developments or fixes come to light, we'll update this post. For now, not all problems can be solved. Patience.